CONFIDENCE FINDINGS 

ON THE POTENTIAL OF 

6X2 AXLES

ABSTRACT

This report documents the confidence that North 

American Class 8 trucking should have in specifying and 

buying tractors with a 6x2 axle configuration. The study 

team engaged the entire industry in the data that is

presented here. Thanks to all of those who contributed 

to this important work. 

Trucking Efficiency

Trucking Efficiency is a joint effort between NACFE and 

the Carbon War Room to double the freight efficiency 

of North American goods movement by 2016 through 

the elimination of market barriers to information, 

demand and supply.

© 2013 North American Council for Freight Efficiency. All rights reserved.

The contents of this document are provided for informational purposes only and do not constitute an endorsement 

of any product, service, industry practice, service provider, manufacturer, or manufacturing process. Nothing 

contained herein is intended to constitute legal, tax, or accounting advice, and NACFE assumes no liability for use of 

the report contents. No portion of this report or accompanying materials may be copied, reproduced or distributed in 

any manner without express attribution to the North American Council for Freight Efficiency.
Confidence Report on 6x2 Axles

1/13/2014 Page 1

Acknowledgements:

Study Team:

John Fehring, Hoosier Performance Engineering LLC, NACFE Program Manager

Suzanne Slick, NACFE Sr. Researcher

Bruce Stockton, Stockton Solutions, External Advisor on Fleet Operations

Mike Roeth, NACFE Executive Director

Study Editor:

Tessa Lee, Carbon War Room

Study Sponsors:

Dana Holding Corporation

Meritor, Inc.

In-Kind Contributions:

Bridgestone Corporation

Michelin North America, Inc.

Performance Innovation Transport, FP Innovations 

NACFE Technical Advisory Committee:

Tim Dzojko, Air Products

Allen Nielsen, CR England

Yves Provencher, FPInnovations

Joe Gold, Frito Lay

Adam Schmieg, Ryder Systems

Bruce Stockton, Stockton Solutions

Dale Spencer, UPS

Steve Phillips, Werner Enterprises

Mike Roeth, NACFE Executive Director
Confidence Report on 6x2 Axles

1/13/2014 Page 2

Table of Contents

Executive Summary...................................................................................................................4

1 Introduction ..................................................................................................................8

1.1 Confidence Reports................................................................................................................ 10

2 Methodology ...............................................................................................................12

2.1 Primary Study Questions........................................................................................................ 13

3 Overview of Currently Available 6x2 Axle Systems for Tractor Trailers............14

3.1 History of 6x2s....................................................................................................................... 14

3.2 6x2 System Details................................................................................................................. 15

3.3 System Manufacturers........................................................................................................... 16

3.3.1 Dana Holding Corporation .............................................................................................. 16

3.3.2 Meritor, Inc. ................................................................................................................... 18

4 Fuel Economy Testing and Data....................................................................................19

4.1 Previous Findings on 6x2s ...................................................................................................... 19

4.1.1 Adoption Rates............................................................................................................... 19

4.1.2 The 6x2 “Package”.......................................................................................................... 19

4.2 New Findings on 6x2s............................................................................................................. 20

4.2.1 Various Test Procedures................................................................................................. 21

4.2.2 Industry Tests Provided to Trucking Efficiency for this Confidence Report....................... 22

4.2.2.1 OEM #1 –Volvo Global Truck Technology.................................................................... 22

4.2.2.2 OEM #2 – Daimler Trucks North America .................................................................... 23

4.2.2.3 Fleet #1 – Con-way Truckload ..................................................................................... 24

4.2.2.4 Fleet #2 – Fleet XXX .................................................................................................... 24

4.2.2.5 Fleet #3 – UPS............................................................................................................. 24

4.2.2.6 Fleet #4 – Nussbaum .................................................................................................. 25

4.2.2.7 Fleet #5 – Fleet YYY..................................................................................................... 25

4.2.3 NACFE-Conducted Tests ................................................................................................. 25

4.3 Test Results: Fuel Economy Improvements............................................................................. 27

5 Other Benefits of 6x2s..................................................................................................29

5.1 Weight................................................................................................................................... 29
Confidence Report on 6x2 Axles

1/13/2014 Page 3

5.2 Cost ....................................................................................................................................... 29

5.3 Maintenance.......................................................................................................................... 30

5.4 Stability Improvement............................................................................................................ 30

6 Adverse Consequences / Challenges.............................................................................32

6.1 Traction ................................................................................................................................. 32

6.2 Tire Wear............................................................................................................................... 34

6.3 Resale Value........................................................................................................................... 36

6.4 Driver Acceptance.................................................................................................................. 36

7 Original Equipment Manufacturers’ Perspectives on System Integration .......................38

8 Summary of Fleet Perspectives and Experiences with 6x2 Configuration .......................40

8.1 Early-Adopting Fleets: ............................................................................................................ 40

8.2 Evaluating Fleets.................................................................................................................... 41

8.3 Fleet Forum Internet Survey................................................................................................... 42

9 Payback Calculator.......................................................................................................44

10 Fleet Implementation Best Practices.............................................................................45

11 Perspectives on Future Axle Products...........................................................................46

12 Conclusions and Recommendations..............................................................................47

13 References...................................................................................................................49

13.1 Media & News Articles........................................................................................................... 49

14 Appendix A: PIT Test Report........................................................................................51
technologies, the North American 

Council for Freight Efficiency (NACFE) 

has partnered with the Carbon War Room 

(CWR) to form the Trucking Efficiency 

Operation. The Operation’s work has 

begun with a series of Confidence 

Reports, of which this report on 6x2 

axles is the second. 

This report focuses on these axle 

technologies because they stand 

to increase fuel efficiency by 2.5%. 

Specifically, this report documents the 

confidence that the North American 

Class 8 trucking industry should have in 

whether or not it is economically rational 

for them to specify and buy tractors with 

a 6x2 axle configuration. Based on the 

claims of 6x2 manufacturers, along 

with some anecdotal data, NACFE’s 

initial research into a variety of fuelefficiency technologies suggested that 

6x2 systems will offer cost-effective fuel 

savings to the vast majority of fleets. But 

although such single-axle drive tractors 

are widely used in European trucking 

and have been for some time, their 

The fuel costs faced by the tractortrailer industry have been swiftly and 

steadily rising over the past decade. 

By 2012 diesel fuel costs reached 

$0.64 per mile, surpassing even the 

combined cost of wages and benefits 

for the drivers. This recent surge in fuel 

prices has reshaped the economics of 

trucking, and the industry is in need of 

solutions if it is to stay profitable.

Fortunately, a myriad of technologies 

which show strong potential for 

achieving cost-effective gains in 

fuel efficiency for Class 8 trucks are 

readily available on the market today. 

Unfortunately, the industry’s uptake of 

such technologies has been stymied by 

a multitude of barriers, central among 

those being a lack of data about the 

true performance gains offered by 

these technologies, and, what’s more, 

a lack of confidence in the data that 

does publically exist today. In order to 

overcome those barriers and facilitate 

the industry’s trust in and adoption 

of the most promising fuel-efficiency 

TRUCKING EFFICIENCY EXECUTIVE SUMMARY:

Confidence Findings on the Potential of 6x2 Axles Report

penetration into the North American 

market thus far has been very slow, and 

today they make up only 2.3% of new 

line haul tractor sales.

In looking into possible reasons for such 

low adoption rates, the aforementioned 

market barrier of a lack of data and 

confidence was found to be particularly 

applicable to the situation of 6x2 systems. 

E F F I C I E N C Y

TRUCKING

The test findings, which are explored in depth in the report, are summarized in this table:

Fuel Economy Test Results

BY 2012 

DIESEL FUEL COSTS 

REACHED $0.64 PER MILE, 

SURPASSING EVEN THE 

COMBINED COST OF 

WAGES AND BENEFITS FOR 

THE DRIVERS.
place, side-by-side, making the findings 

of this report not only more compelling 

but also more likely to be relevant to a 

range of fleets and truck owners.

Seven sets of existing testing data, five 

from fleets and two from OEMs, are 

presented in this report. That data is 

supported by three additional sets of 

track tests on 6x2 performance that 

NACFE commissioned in the course 

of conducting its research. Along 

with the test results themselves, 

this report supplies details on the 

testing protocols used in each 

case, so that readers can better 

judge which tests are most 

applicable to their own needs 

or duty cycles.

A d d i t i o n a l i n t e r v i e w s 

were conducted with 6x2 

technology manufacturers and 

other stakeholders in the sector 

to round out the conclusions 

drawn from this test data.

By analyzing these various sets of 

test data, Trucking Efficiency finds 

that fleets can be confident that they 

will enjoy about a 2.5% reduction in 

fuel use, and payback on investment in 

about 20 months, by adopting currently 

available 6x2 axle technologies.

In sum, this report places the 6x2 

technology near the top-right quadrant 

of the Trucking Efficiency Confidence 

Matrix, giving it a medium-to-high 

Confidence Rating – meaning that 

Trucking Efficiency is confident in that 

the existing data, given its the breadth 

Numerous market research studies, 

including some conducted by NACFE, 

suggest that fleets trust the experience 

of other fleets more than any source of 

information besides their own. And yet, 

prior to the publication of this report, 

there was very limited data on the actual 

experience of early-adopting fleets with 

6x2 systems. 

E F F I C I E N C Y

TRUCKING

For more information, contact 

Mike Roeth at 260.750.0106, mroeth@carbonwarroom.com and/or visit www.nacfe.org 

and accuracy, does demonstrate that 

6x2s will have an acceptably short 

payback period as to justify their 

widespread adoption by the industry:

This Confidence Rating indicates that 

Trucking Efficiency is highly confident that 

6x2 axle configurations offer significant 

fuel-efficiency gains. Besides fuel-savings, 

other benefits of 6x2s include weight 

reduction, lower maintenance costs and 

improved truck stability. 

Along with discussing the benefits of 

6x2s, this report catalogues the concerns 

most commonly cited as reasons not 

to adopt 6x2s, including challenges of 

traction, overall tire wear, resale value, 

and driver acceptance. These are indeed 

justified concerns. However, this report 

considers each concern individually, and 

concludes that the benefits of 6x2s today 

outweigh these concerns for a variety of 

reasons. For example, new technologies 

Payback and Confidence

TRUCKING EFFICIENCY EXECUTIVE SUMMARY:

Confidence Findings on the Potential of 6x2 Axles Report

BY ANALYZING THESE 

VARIOUS SETS OF TEST DATA, 

TRUCKING EFFICIENCY FINDS 

THAT FLEETS CAN BE CONFIDENT 

THAT THEY WILL ENJOY ABOUT 

A 2.5% REDUCTION IN FUEL USE, 

AND PAYBACK ON INVESTMENT IN 

ABOUT 20 MONTHS, BY ADOPTING 

CURRENTLY AVAILABLE 6X2 AXLE 

TECHNOLOGIES.

Thanks to an extensive series of 

interviews, and permission from the 

industry to publish their preexisting test 

results, this confidence report marks the 

first and only place where the actual 

data of various fleets’ experiences with 

6x2s is shared publically. This report also 

represents the first time that a variety 

of different sources of data and testing 

results are collected and shared in one 
here are relevant to or replicable in their 

own operations.

Future research should attempt 

to verify some initial findings, 

also discussed in the report, 

around the much greater 

potential efficiency gains 

offered by installing a 

suite of complementary 

technologies, which 

Trucking Efficiency 

has dubbed the “6x2 

package.” The report 

also gives some initial 

findings of the impact of 

6x2 axles on tire wear, but 

additional test data is needed 

to strengthen those numbers.

Additionally, Trucking Efficiency is 

always seeking to expand the data 

or case studies that we can provide to 

the industry. We invite you to share with 

us your own experiences with 6x2 axle 

systems – whether you have adopted 

them already, are considering doing so, 

or have chosen not to pursue them at 

this time.

It is the hope of the Trucking Efficiency 

Operation that this report will catalyze 

significant new interest in the 6x2 axle 

technology as a way to increase fuel 

efficiency and obtain other benefits.

for automatic load shifting, limit the 

traction difference between a 6x2 and 

a 6x4, and therefore Trucking Efficiency 

recommends such technologies be 

automatically specified for any 6x2 

tractor.

To complement these new figures for the 

average predicted efficiency gains offered 

by 6x2 axle systems, this Confidence 

Report includes a Payback Calculator, 

with which individual fleets or truck 

owners can more accurately predict 

the gains that they themselves would 

experience with a 6x2 configuration. 

Users input various figures and details 

specific to their operations, and duty 

cycles, and receive a tailored picture of 

whether or not 6x2 axles are going to be 

right for their operations. 

Unlike many other such calculators, the 

Payback Calculator presented in this 

Confidence Report considers both the 

benefits or savings and the challenges 

or costs posed by 6x2s in determining 

its results. The goal of the Payback 

Calculator is not to provide complete and 

thoroughly accurate data of how 6x2s will 

perform for a given fleet, but rather to 

suggest whether users should seriously 

explore the technology for themselves, 

perhaps by specifying on a few of their 

trucks, or otherwise investing in their 

own rounds of testing.

Along with the results of the Payback 

Calculator, fleets can use this Confidence 

Report as an initial decision-making 

tool by considering whether the best 

practices for 6x2 performance detailed 

E F F I C I E N C Y

TRUCKING

For more information, contact 

Mike Roeth at 260.750.0106, mroeth@carbonwarroom.com and/or visit www.nacfe.org 

TRUCKING EFFICIENCY EXECUTIVE SUMMARY:

Confidence Findings on the Potential of 6x2 Axles Report

TO COMPLEMENT THESE 

NEW FIGURES FOR THE AVERAGE 

PREDICTED EFFICIENCY GAINS 

OFFERED BY 6X2 AXLE SYSTEMS, THIS 

CONFIDENCE REPORT INCLUDES A 

PAYBACK CALCULATOR, WITH WHICH 

INDIVIDUAL FLEETS OR TRUCK OWNERS 

CAN MORE ACCURATELY PREDICT 

THE GAINS THAT THEY THEMSELVES 

WOULD EXPERIENCE WITH A 6X2 

CONFIGURATION.

About Operation 

Trucking Efficiency

Operation Trucking Efficiency is 

a joint effort between NACFE and 

the Carbon War Room to double the 

freight efficiency of North American 

goods movement by 2016 through 

the elimination of market barriers to 

information, demand and supply.

Worldwide, the heavy-duty freight trucks 

emit 1.6 gigatons of CO2 emissions annually 

– 5.5% of society’s total greenhouse gas 

emissions. These emissions are the result 

of the trucking sector’s dependence on 

petroleum-based fuels. From a global 
perspective, truck manufacturing is a 

growth market which will likely see up to 

33 million new units built by 2015. But this 

growth, though profitable, could result in 

massive increases in trucking’s emissions 

– unless the trucking sector improves its 

fuel efficiency as fast as it expands.

With fuel prices continuing to rise, the 

adoption of efficiency technologies by all 

classes of trucks and fleets offers significant 

cost-savings to the sector while reducing 

emissions. For example, for a typical 

heavy-duty truck in the United States, a 

5% reduction in fuel-use gained through 

improved efficiency offers yearly savings 

of over $4,000. Technologies capable of 

conferring such gains are relatively cheap 

to implement and widely available on the 

market. Many have the potential to be 

retrofitted onto existing trucks. 

But in spite of the potential cost savings, 

even the most promising of these 

technologies are not yet being widely 

adopted by the North American trucking 

industry. Operation Trucking Efficiency 

finds that the following market barriers 

are responsible for this: 

Lack of Confidence in the Data on 

Efficiency Technologies – New 

technologies abound, but fleet owners 

lack cross-comparable, credible, and 

widely available data proving their 

potential performances. Often the only 

existing data are producer claims, which 

fleets put low trust in. Fleets worry that 

savings will be less than promised, and 

that technologies will negatively impact 

their operations. 

the Carbon War Room aims to create 

well-functioning, high-growth, and 

low-carbon marketplaces by launching 

Operations in those sectors. The War 

Room’s current Operations include 

Maritime Shipping Efficiency, Green 

Capital for Energy Efficiency in the Built 

Environment, Renewable Jet Fuels, 

Smart Island Economies, and Trucking 

Efficiency. 

For more information, please visit 

www.carbonwarroom.com.

About NACFE

The North American Council for Freight 

Efficiency will drive the development 

and adoption of efficiency-enhancing, 

environmentally-beneficial, and costeffective technologies, services, and 

methodologies in the North American 

freight industry by establishing 

and communicating credible and 

performance-based benefits. The Council 

is an effort of fleets, manufacturers, 

vehicle builders and other government 

and non-government organizations 

coming together to improve North 

American goods movement. 

More can be learned about the NACFE at 

www.nacfe.org. 

About the Carbon 

War Room

The Carbon War Room is a global 

nonprofit, founded by Sir Richard Branson 

and a team of like-minded entrepreneurs, 

that accelerates the adoption of business 

solutions that reduce carbon emissions at 

gigaton scale and advance the low-carbon 

economy. The organization focuses 

on solutions that can be realized using 

proven technologies under current policy 

landscapes.

Working collaboratively in sectors 

where we have proven that profitable 

emission-reduction opportunities exist, 

E F F I C I E N C Y

TRUCKING

NACFE

TRUCKING EFFICIENCY EXECUTIVE SUMMARY:

Confidence Findings on the Potential of 6x2 Axles Report

Information is Not Shared – When fleets 

do independently test a technology, 

the tests are expensive and timeconsuming, leading to 18-month 

average implementation times and low 

purchase quantities. Fleets tend to test 

in parallel, rather than sharing their test 

results or otherwise collaborating in 

obtaining performance data, resulting 

in an unnecessary duplication of cost 

and effort. 

This Confidence Report series from 

Operation Trucking Efficiency was born 

out of not only the identification of 

these barriers, but also conversations 

with the industry, which made it clear 

that the elimination of these barriers 

requires a credible and independent 

source of information on fuel efficiency 

technologies and their applications. The 

Confidence Reports aim to serve of the 

first such source on the 

For more information, contact 

Mike Roeth at 260.750.0106, mroeth@carbonwarroom.com and/or visit www.nacfe.org 
Confidence Report on 6x2 Axles

1/13/2014 Page 8

1 Introduction

This report forms part of the continued work of Trucking Efficiency, a joint initiative from the North 

American Council for Freight Efficiency (NACFE) and the Carbon War Room (CWR), highlighting the 

potential of fuel-efficient technologies and practices in over-the-road goods movement.

The fuel costs faced by the tractor-trailer industry have been swiftly and steadily rising over the past 

decade (Figure 1). By 2012, as Figure 2 shows, fuel costs had reached $0.641 per mile, surpassing even 

the costs for the driver (wages plus benefits).

Figure 1: US Annual Diesel Fuel Prices

Figure 2: Trucking Operational Cost

Source: American Transportation Research Institute 2012. Operational Costs of Trucking. 

$1.51 

$1.81 

$2.40 

$2.71 

$2.89 

$3.80 

$2.47 

$2.99 

$3.83 $3.97 

 $-

 $0.50

 $1.00

 $1.50

 $2.00

 $2.50

 $3.00

 $3.50

 $4.00

 $4.50

2004 2005 2006 2006 2008 2009 2010 2011 2011 2012

Average Annual USA Diesel Fuel Price

Source: US Energy Information Administration - Feb '13
Confidence Report on 6x2 Axles

1/13/2014 Page 9

Investment into technologies and practices that allow a truck or fleet to increase their fuel efficiency –

meaning they can do the same amount of business while spending less on fuel – is a hugely promising 

option for the industry in light of this trend. 

To understand, and thereby better facilitate uptake of such technologies, NACFE conducts an annual 

review, “the Fleet Fuel Study,” of the industry-wide adoption rates of over sixty fuel-efficient 

technologies currently available for Class 8 tractors and 

trailers. This work, available on the www.nacfe.org

website, has been called “The most comprehensive 

study of Class 8 fuel efficiency adoption ever 

conducted.” (Truck News, 2012)

In February 2013, NACFE completed the first annual 

update of that study, and, for the first time ever in any 

study, included additional research into the use of fuelefficient products and practices by ten of the largest, 

most data-driven fleets (Figure 3). Those fleets 

represent both regional and long-haul tractors and 

trailers, in both dry goods and refrigerated cargo 

movement, and boast a combined inventory of 41,000 tractors and 125,000 trailers. The 2013 study 

reviewed a decade of those ten fleets’ specific experience with the sixty-plus technologies. Each fleet 

shared the percentage of their new purchases of tractors and trailers which included any of the 

technologies. They also shared ten-years-worth of annual fuel economy data for the trucks in their fleet. 

With these two pieces of information, which will be updated each year for future reports, NACFE is able

to share insights into the following trends with the industry:

 Adoption curves for each of the sixty-plus technologies, indicating which technologies have the 

steepest adoption rates, which are being adopted steadily but slowly, and which are not being 

purchased at all. These curves also show how uniformly (or not) fleets are acting in their 

adoption patterns.

 Identification among the various fleets of the innovators, early-majority or late-majority 

adopters, and even laggards in new technology adoption as described specifically in the report.

 Comparison of technology adoption to overall fuel efficiency. 

The overriding take-away from this study is that fleets are enjoying dramatic improvements in their fuel 

efficiency by adopting the various technologies surveyed - saving about $5,700 per tractor per year in 

fuel costs compared with a fleet who has not invested in any efficiency technologies.

Figure 3: NACFE Fuel Study Participating Fleets
Confidence Report on 6x2 Axles

1/13/2014 Page 10

1.1 Confidence Reports

The Fleet Fuel Studies provide useful insights into adoption trends in the industry and the practices of 

different major fleets, and NACFE hopes that this information could spur additional investment, 

particularly by fleets that may be lagging behind the overall industry when it comes to certain widelyadopted technologies. However, in the course of conducting the studies, it was clear that some 

technologies which show strong potential for achieving cost-effective gains in fuel efficiency are only 

being adopted by the most progressive or innovative of fleets. In order to facilitate the wider industry’s 

trust in and adoption of such technologies, NACFE and the CWR formed the Trucking Efficiency

operation. The operation has begun a series of reports, called “Confidence Reports,” which will take an 

in-depth look at those technologies one-by-one. 

6x2 axles are one such technology that shows limited initial adoption rates in spite of promising results, 

and therefore 6x2 axles were found to meet Trucking Efficiency’s criteria for a technology to investigate 

further. Specifically, the NACFE Technical Advisory Committee (TAC) found that this technology should 

be a priority for additional research due to the fact that rarely does a single technology offer fuel-saving, 

weight-reduction, and cost-neutrality or even cost-savings – and initial data on 6x2s find that they can 

offer all three benefits. Other benefits of 6x2s include lower maintenance costs and improved truck 

stability. The challenges associated with the adoption of 6x2s that NACFE found in its initial 

consideration of the technology included concerns around traction, overall tire wear, resale value and 

driver acceptance.

This Confidence Report was produced in order to confirm or refute those benefits and challenges, and 

provide more information to the industry on 6x2 axles. This exploration of 6x2 axles is the second in this 

new series of similar studies the first of which was published in August of 2013 on Tire Pressure Systems. 

Confidence Reports provide a concise introduction to a promising category of fuel-efficiency 

technologies, covering the key details of their applications, benefits and variables. The reports are 

produced via a data mining process that combs both public information and otherwise-private

information that is shared with Trucking Efficiency for the purpose of the report, in order to collect an 

unparalleled range of testing data and case-studies on a technology. The information gathered in each 

Confidence Report will typically include:

 Suppliers’ Bench Tests – For instance, predicting fuel economy for vehicle performance.

 Fleet-Reported Case Studies – Best practices and optimal use of technology.

 Truck Manufacturers – How the technology performs in engineering testing, as well as their 

product introduction and ramp plans.

 Public Reports/Test/Data, press information, etc.
Confidence Report on 6x2 Axles

1/13/2014 Page 11

Each Confidence Report is published in tandem with a simple and accurate ‘Payback and Performance 

Calculation Tool,’ that will facilitate a fleet’s adoption of a technology with the confidence that it is

indeed right for them. 

The objectives of this study are the following:

 Understand the history of the development of the 6x2 axle concept and the current 6x2

products available to North American trucking operations. 

 Work with the industry to detail the fuel efficiency performance of the technology. 

 Document both the benefits and adverse consequence of adoption, and outline best practices. 

 Calculate a payback and develop a tool for use by fleets and others to determine their return on 

investment of this technology. 

 Report an industry-wide ‘level of confidence’ that end-users can have in both the existing data 

and actual gains they will enjoy by adopting 6x2 axles.
Confidence Report on 6x2 Axles

1/13/2014 Page 12

2 Methodology

Trucking Efficiency’s approach to Confidence Reports, illustrated in Figure 4, is centered in the

recruitment of an unbiased team of trucking experts who will conduct the work. For this 6x2 study the 

core team included:

 John Fehring, Program Manager and Key Technical Lead, Hoosier Performance Engineering

 Bruce Stockton, Early Adopter and Fleet Perspectives, Stockton Solutions

 Sue Slick, Sr. Researcher, NACFE

 Mike Roeth, Executive Director, NACFE

The NACFE TAC, via Trucking Efficiency, commissioned 

this report in August 2012, and the study team began 

actively working on the project in September of that 

year. The team first spent three months reviewing all 

publically available information on 6x2 axles and creating 

a list of the expected benefits and challenges of adopting 

this technology. Next, the team met multiple times with 

the two primary providers of 6x2 technology (who are 

also sponsors of this study), Dana Holding Corporation

and Meritor Inc., visiting the engineering and validation 

locations of both companies to review the performance, 

cost, and weight of their 6x2 axles, and any recent 

improvements to their configurations. This effort 

included the completion of a thorough data-collection document on the offerings from both companies 

with regard to their key customer attributes. It also included a review of the performance predictions 

each company had developed, using the manufacturers’ simulation methodologies given the bench 

testing of the axles assuming various duty cycles. 

The team next moved to the tractor original equipment manufacturers (OEMs) and the fleets 

themselves, building on their research into public and manufacturer data by gathering private and 

confidential information, both qualitative and quantitative. When needed, non-disclosure agreements 

were executed in order to protect information while still allowing the team to utilize it in generating 

their findings and recommendations. The team conducted in-person and over-the-phone interviews 

with various tractor OEMs and end-user fleets, as well as an online survey of fleet’s perspectives on 6x2

axles. The fleets interviewed included innovative first buyers, early adopters, and those considering 

adoption. 

Trucking Efficiency also used major truck conferences to obtain additional data and perspectives from

axle manufacturers, end users, and tractor OEMs, including the 2012 CALSTART HTUF National meeting 

Figure 4: Approach to Confidence Reports
Confidence Report on 6x2 Axles

1/13/2014 Page 13

in Charlotte, the 2013 Technology and Maintenance Council exhibition in Nashville, and the 2013 MidAmerica Trucking Show in Louisville. 

Moreover, Trucking Efficiency decided to conduct its own fuel consumption track tests for 6x2s. These 

tests were conducted in June 2013 with the assistance of FPInnovations’ Performance Innovation 

Transport (PIT) group, using their semi-annual Energotest procedures. PIT is a Canadian-based, fleetmember organization conducting research and testing for trucking efficiency. Equipment for those tests 

was provided with the help of Meritor, Volvo, and a major Canadian fleet. 

Overall, in the course of this 6x2 study, a significant amount of new data was created, while previously 

private information was shared here for the benefit of the entire industry. In total, ten independent and

robust fuel efficiency test reports are included in this study - three from Trucking Efficiency itself, two via

the engineering departments of the tractor OEMs, and five from fleets’ own rigorous testing and data 

collection processes, giving the study team confidence that the combined results of the testing data 

accurately isolated and reflected the impact of the 6x2 axles on fuel efficiency. 

In crafting this Confidence Report the team analyzed all of the information, formulating fuel efficiency 

predictions, best practices for adoption, and more, and culminating in the development of a projected 

payback calculator and a confidence rating for fleets to use as they consider specifying 6x2 axles on their 

future tractor orders. 

2.1 Primary Study Questions

A series of primary study questions was created to help guide the study team’s effort:

 What 6x2 technologies are currently (or soon-to-be) available on the market, and what 

distinguishes each of them?

 Which of the various 6x2 systems have been adopted to date, and in what volumes?

 What degree of improvement in fuel efficiency (fuel economy) were seen with 6x2s?

 What were the adoption rates of 6x2s among the study fleets? 

 Who is considering adoption, and what additional information do they need to decide whether 

or not to specify and buy 6x2s?

 What were the key attributes of the technology which attracted the very early adopters? 

(Mesilla Valley Transport, Con-way Truckload, etc.)

 Which fleets have been successful using 6x2s and why?

 Why did some fleets test 6x2s but then decide not to pursue purchasing them?

 Do the axles perform well?

 What is required to make 6x2s work well? (i.e. complementing features, components or 

systems?) Are tractor OEMs supporting adoption by making axles and accompanying features

available?

 How did the early adopters handle traction issues?

 What other challenges are current adopters facing with these technologies, and how are they 

overcoming them? 
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3 Overview of Currently Available 6x2 Axle Systems 

for Tractor Trailers

3.1 History of 6x2s

As James Menzies pointed out in his Truck News December 2012 article “The 6x2s are coming” (Menzies, 

DAILY NEWS Dec 7, 2012 4:04 PM, 2012), truck OEMs and suppliers have successfully saturated the 

market with the simplest productivity improvements for fleet owners and owner/operators. The article

asked, essentially, what more could easily be done to increase payload and/or improve fuel economy, 

given that all the ‘low-hanging fruit’ has been picked – 6x2s were posited as one likely answer.

Many ideas and technologies for improving fuel economy were brought to the trucking industry back in 

the 1970s, during that era of high fuel prices, and some of those are being given a second look today. 

The industry realizes that fuel prices are likely to be both volatile and high for years to come, and this 

fact, combined with increasing legislation surrounding CO2 and other emissions, has brought a new focus 

to the fuel economy of heavy trucks. The 6x2 configuration is one of those older technologies to be 

gaining renewed interest, due to its potential weight savings and fuel economy improvements. 

In spite of not being a new technology, 6x2 systems are still not widely used in North America and are 

considered by many to be an “odd-ball spec.” Until recently, truck buyers had to request a 6x2 axle 

system through a Special Equipment (SE) order at the OEM, which can increase the lead time for a new 

truck’s delivery. Most OEMs now offer the 6x2 in regular production as optional equipment.

The 6x2 configuration has been the standard configuration for over-the-road trucks in many parts of 

Europe for the last few decades. Many such technological advancements from Europe are finding their 

way to the North American market of late. For example, European-originated advances in electronics 

and controls are bringing the operating performance of a 6x2 up to par with that of a 6x4 and require

minimal driver interaction and close-to-zero operational differences. 

As trucking continues to be commoditized, carriers must do everything possible to increase their 

equipment’s productivity and reduce their operating costs. (Menzies, Truck News Sep 4, 2012) Several 

fleets have aggressively adopted the 6x2 configuration, equipping the majority of their trucks with this 

system. These fleets, along with some “hyper-mileage” owner/operators, are adopting 6x2s as a means 

to bolster their bottom lines thanks to the additional fuel economy and payload that the systems allow.

Together, these early adopters are increasing the penetration of 6x2s in the North American trucking 

market, dispelling many of the concerns, especially concerns over traction, that have limited the 

adoption of 6x2s thus far, and improving the resale values of 6x2 trucks in the process.
Confidence Report on 6x2 Axles

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3.2 6x2 System Details

A typical Class 8 tractor today is equipped with three axles - a forward, unpowered, steer axle, and two 

rear-drive axles which are often referred to as a “live tandem” axle system. This arrangement is 

commonly called a 6x4 configuration, given that it involves six wheel-ends on the tractor (two on each 

axle), with the four rear wheel-ends all being powered. A 6x2 configuration still has the same three 

axles and same six wheel-ends, but only one of the rear tandem axles (2 wheel-ends) is actually ‘driven,’ 

or powered by the engine. The non-driving rear axle is often referred to as a “dead” axle, and the whole

6x2 system is often called a “dead axle tandem.” 

There are two possible configurations of the 6x2 system, differentiated by the location of the nondriving rear axle. The first and most common 6x2 system is the “tag tandem” where the forward-rear 

axle is driven and the rear-rear axle is the “non-driver” or “tag” axle. Alternately, a “pusher tag” 6x2 

system has the non-driven axle in the forward-rear location and the drive axle in the rear-rear location. 

6x2 systems are available in the same 40k and 46k tandem ratings (referring to the maximum weight in 

thousands of pounds that the two axles together can haul) commonly found in 6x4 arrangements, with 

the dead axle, whether forward-rear or rear-rear, designed to carry the same load as the driven axle. 

However, since all of the drive and retarding engine torque passes through a single rear axle (the driven 

one), 6x2 system manufactures have either specified the use of ring and pinion gear sets with a 23k max 

haul weight per axle (not 20k), or have developed gears and ratios specific to their 6x2 systems.

Manufacturers do not recommend conversion from a 6x4 to a 6x2 arrangement by simply removing the 

interaxle drive shaft and gear sets for the non-driven axle, due to the increased torque that will pass 

through the single driven axle gear set. (That is, a 6x4 system distributes the torque across both axles 

and their gear sets. A 6x2 system uses slightly stronger or larger components to handle the more 

concentrated torque.) 

The lack of internal gearing in the 6x2 configuration (as the dead axle does not need any drive gears) 

prevents fuel losses from the internal frictional and/or lubricant churning inherent in the use of any 

Figure 5: 6x2 Tag Axle Tractor Diagram

Forward steer axle –

identical in both a 6x4 

and a 6x2 configuration

Rear drive “tandem” 

axles, one of which is not 

powered in a 6x2 

configuration
Confidence Report on 6x2 Axles

1/13/2014 Page 16

gears, and thereby gives 6x2 systems higher fuel efficiency than 6x4s. 6x2 systems also weigh less than 

6x4s systems, conferring additional fuel savings.

Today’s dead axles utilize the same wheel-end equipment as the drive axles in the 6x2 system, meaning 

that their hubs and brake components are identical, which simplifies maintenance. Some 6x2

manufacturers use a drive axle for their dead axle’s housing and simply plug or cover the axle ports, 

carrier openings and input shaft openings. Others have developed dedicated square or round dead axle 

housings, similar to those used in trailer applications. 

Other component changes that can complement a 6x2’s performance and efficiency should be 

considered when specifying a 6x2 vehicle configuration. For instance, engine-, transmission- and finaldrive ratios may be modified to optimize the drive train for a “gear fast, run slow” setup. Additionally, 

trailer tires may be used on the dead axle, as their tread pattern offers reduced rolling resistance 

compared to lugged tractor drive tires. Wide Base Single Tires (WBS) could be specified along with the 

6x2, as these can offer up to an additional 400 lbs. in weight savings and reduced rolling resistance, both 

of which improve fuel efficiency. 

While it is accepted that 6x2s reduce weight, and generally accepted that they otherwise improve fuel 

efficiency, one of the major barriers to their wide-spread adoption in the US is a lingering concern over 

whether or not they provide adequate traction. Many inside the trucking industry equate the 

performance of a 6x4 tractor to that of a 4x4 pickup and the performance of a 6x2 to that of a 2WD 

pickup. But this perception is inaccurate. Even the earliest 6x2 systems incorporated manual dump 

valves that allowed the truck driver to momentarily transfer load from the dead axle to the driven axle 

during low traction conditions. Several manufacturers have since developed automated load-transfer 

systems that shift the load from the dead axle to the driven axle during low-traction moments without 

requiring driver interaction. In sum, thanks to these systems and as detailed later in this report, data on 

6x2s suggests that these actions significantly reduce the traction issue inherent with having one less 

drive axle. Several truck OEMs are now integrating such load-transfer controls directly into their overall 

vehicle controls systems for vehicles specified with 6x2s.

3.3 System Manufacturers

Two companies, Dana Holding Corporation and Meritor, Inc., currently produce 6x2 systems for the 

North American market. Both of their systems have the forward-rear axle serving as the driven axle in 

their systems. The following contains product descriptions supplied directly from each manufacturer. 

More information and data can be reviewed via each manufacturers’ website.

3.3.1 Dana Holding Corporation

In the spring of 2013, Dana Holding Corporation introduced its newest 6x2 system, the Spicer® 

EconoTrekTM tandem. Like all 6x2s, this newest system improves fuel economy through decreases in 

mechanical drive-train energy loss. It also reduces overall weight by up to 450 pounds compared to a 

traditional 40,000-pound tandem axle arrangement. 
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The Spicer® EconoTrek tandem axle system combines the company’s newly upgraded Spicer® S170 or 

S190 single-reduction single-drive axles (the driven axle in the 6x2) with a new, lightweight Spicer S20-

045B that serves as the non-driven or “tag” axle in the system.

First introduced in 2004, Spicer® S170 and S190 are high-performance single-drive axles designed for 

efficiency. Their newest iterations within the Spicer® EconoTrek system have improved durability, 

meaning less maintenance is required. 

Manufacturer Dana boasts that these newest 

Spicer® S170 and S190 drive axles are more 

resistant to differential spin-out events than 

ever before.

The new Spicer® S20-045B tag axle also 

features advanced designs leveraged from 

Dana’s popular line of Spicer® commercialvehicle drive axles. Made of high-strength, 

micro-alloy steel with a thin-wall, squareedge-formed axle housing box, it weighs 20 

to 30 pounds less than competitive models of 

tag axles. The Spicer® S20-045B axle is shaped like a low-profile drive axle, which provides significant 

strength to the axle’s vertical loadbearing capabilities, and also reduces deflection to limit tire wear 

while simplifying suspension and chassis installation.

The Spicer® EconoTrek tandem axle system is available with SelecTTracTM for use with wide-based single 

tires, and is also optimized for use with electronic-controlled air suspension systems, such as the Bendix 

eTracTM system. The eTracTM system serves to transfer weight to the Spicer® S170 or S190 drive axles in 

the 6x2 system to allow for improved low-speed traction, when combined with the wheel-differential 

lock feature.

Dana previously offered 6x2 configurations that 

also featured a Spicer® S170 or S190 single drive 

axle used in combination with a Spicer® S21-060B 

or S23-070B tag axle. The Spicer® S21-060B and 

S23-070B tag axles provided common torque rod 

brackets and axle seats for air-ride suspensions, as 

well as a common wheel-end configuration to 

support a wide variety of brake and hub options. 

These tag axles utilized standard drive-axle 

housings with removable bolted cover plates that could accept a drive-axle carrier assembly and axle 

shafts, allowing them to be converted into driven-axles in OEM pre-approved applications.

Figure 6: Dana's Spicer EconoTrek Tandem Axle with Welded Bowl 

Tag Axle

Figure 7: Dana's Bolted Bowl Tag Axle
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3.3.2 Meritor, Inc.

Meritor, Inc. specializes in commercial-vehicle components for the trucking industry, and their 

commercial literature states that they “pride themselves in developing solutions that are known for not 

only bulletproof reliability but also efficiency.”

One such solution is their FUELiteTM 6x2 

system. The FUELite tandem axle set is 

the first member of the Meritor 6x2 

configuration axle family, the SoloDrive 

Series. Based on their 160-Series drive 

axle, and named after the two key 

benefits it delivers, the FUELite 6x2 

system offers 400lbs of weight savings 

compared to a similar 6x4 system, and 

provides an estimated two percent fuel

efficiency increase. 

Features of the drive axle within the 

FUELite system include: Meritor DualTracTM housing that allows the 6x2 system to use either traditional 

duals or wide-based single tires, driver-controlled diff lock, and available ratios ranging from 2.50-4.10 

and a 12.7 millimeter wall housing that’s compatible with all current 40,000-pound tandem hook-ups. 

The FUELite system also includes a new, lightweight, and unique rectangular tag axle designed for ease 

of OEM installation, parts commonality, and reliability. Features of the tag axle include R-series spindles, 

an integral upper torque rod bracket designed to accommodate all 40K line haul suspensions, and a 

12.7mm thick wall housing.

The FUELite 6x2 system is compatible with various traction-management systems available today.

Meritor recommends that customers always spec their 6x2 systems in a package with enhanced traction 

controls and a locking differential. Advanced traction-management systems, such as Meritor WABCO’s 

electronically-controlled air suspension (ECAS) system, are designed to improve a vehicle's air 

suspension control system by electronically maintaining a set ride height between the axle and chassis 

when road and vehicle characteristics change. ECAS, for example, also offers automatic load transfer, 

significantly reducing drive-axle wheel spin and loss of traction control during periods of aggressive 

acceleration. Advanced traction-management systems, such as Meritor WABCO’s ECAS system,

combined with the efficiency delivered by the FUELite 6x2 axle system, allow a truck to operate more 

fuel-efficiently while still enjoying traction comparable to that afforded by a heavier 6x4 system in all but 

the most extreme situations. 

Figure 8: Meritor’s FUELite Tandem Axle
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4 Fuel Economy Testing and Data

As mentioned earlier, the fuel consumption savings of 6x2 axles are the main benefit of adopting them. 

This chapter will offer significant detail into the fuel savings achieved through specifying this technology 

on Class 8 over the road tractors. Data will include adoption experience and testing completed by 

Trucking Efficiency along with information gathered from axle manufacturers, tractor OEMs and fleets.

4.1 Previous Findings on 6x2s 

NACFE has uncovered previous findings on 6x2 axle configurations via various methods over the past 

few years which are detailed here.

4.1.1 Adoption Rates

As will be detailed later, both primary axle manufacturers, Dana Holding Corporation and Meritor, Inc., 

have 6x2 products available to the industry with data book availability at 5 of the 6 tractor OEMs. 

Daimler, through its Detroit subsidiary, also builds 6x4 axles, but does not currently offer a 6x2 

configuration. 

The 6x2 (and 4x2) has been used extensively in Europe and other parts of the world for many years, yet 

only accounts for about 4% of new line haul tractor sales in the US market. The total US market 

penetration rate of 6x2s remains low at around 2.3%, but is beginning to show months of higher 

production as larger fleets order batches of trucks with the feature.

Adoption of 6x2 axles to date in NACFE’s Fleet Fuel Study has been dominated by one of the fleets, Conway Truckload, who has purchased around 95% of their new tractors in the past few years with 6x2s,

and after ramping up over the past five years now predominately runs trucks with tandem tag 6x2 axles. 

A few other fleets have only purchased and tested small numbers, but reported that they are 

considering much larger future purchases.

4.1.2 The 6x2 “Package”

During the course of its 2012 and 2013 Fleet Fuel Studies, NACFE uncovered that, although 6x2 systems

can be adopted singularly, many early-adopting fleets are choosing to purchase them as part of a

specific package of complementary features, what NACFE is calling the “6x2 package.” The typical 6x2 

package, in addition to the 6x2 tandem axle configuration itself, can include regular wide-base single 

(WBS) tires on the drive, ribbed low-rolling resistance WBS trailer tires on the tag axle, a direct-drive

(DD) transmission, and a “gear fast, run slow” rear-axle ratio approaching 2.50 and filled with lowviscosity synthetic lube. Moreover, engine OEMs are beginning to optimize their own calibrations 

around much lower engine cruise speeds. Where 1350 rpms at a cruise speed of 64 mph was once the 

norm, engine speeds are being lowered to run at 1200, 1150, or even lower. To complement this trend,

axle manufacturers are working to provide lower gear sets for the single drive axles in the 6x2 tandems.
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In 2010, noticing the trend towards the adoption of 6x2 axles within this 6x2 package, NACFE drafted a

quick investigation of the benefits of such a package (NACFE website – 6x2 Executive Report), and found 

that fleets running the aforementioned 6x2 package were reporting a 0.4 to 0.5 mpg fuel efficiency 

improvement. A 0.4 mpg improvement on a 6.5 mpg base tractor represents an over 6% improvement in 

total fuel consumption – in fact it was found that this package is delivering approximately a 6.5% 

improvement over a standard 6x4 configuration with dual-tires and overdrive transmission. 

From these figures on the gains offered by the 6x2 package, some in the industry have anecdotally 

rationalized that each individual technology offers 1/3 of the benefit – 2% for the 6x2 axle system, 2% 

for the wide base tires, and 2% for the direct drive transmissions. However, it is actually quite difficult to 

convert data on technologies in packages to data on technologies working alone, or vice versa. For 

example, if any single improvement to a truck’s aerodynamic shape offered a 3% gain, four such 

improvements might only offer a 10% gain, and not a 12% gain, as a given area of aerodynamic drag can 

only be reduced/removed once, but multiple technologies in the package might each improve the 

airflow over that same area. 

Therefore, in light of the inability to make firm predictions about the performance of a 6x2 system based 

on data about technology packages, this current 6x2 Confidence Report from Trucking Efficiency seeks 

to investigate and confirm the performance benefits of the 6x2 axle only.

This study hypothesizes that the primary benefit offered by a 6x2 axle system is an improvement in fuel 

economy. A 6x2 system allows for the removal of one drive axle’s gear set, the inter-axle differential,

inter-axle drive shafts and axle shafts, thereby significantly reducing the frictional and churning losses 

associated with those components. It is also possible to put trailer tires onto a tag axle, reducing rolling 

resistance and obtaining an incremental improvement in fuel economy, not to mention lowering the

cost of a retread in that position.

4.2 New Findings on 6x2s 

The Confidence Report study team worked with the industry to gather existing 6x2 comparison fuel 

economy test results. The goal was to have a significant set of data in order to confidently predict the 

fuel efficiency benefits of a 6x2 system. 

The study team was ultimately able to accumulate robust test sets from two OEMs and five major fleets, 

in addition to the data from three new tests conducted for the purposes of this report by NACFE in 

conjunction with PIT in Canada. Finally, data was collected from the two 6x2 axle manufacturers, Dana 

Holding Corporation and Meritor, Inc.. 

The OEM and NACFE tests are the typical SAE/TMC Type II or Type III fuel economy tests, while all of the 

fleet data represents the results of monitoring real-world freight operations, with data collected over 

periods ranging from one month to twelve months. The manufacturers used simulation tools to predict 

the fuel economy of various duty cycles, using axle bench test efficiency data as their primary input. 
Confidence Report on 6x2 Axles

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Predictions from the axle manufacturers of the 6x2’s fuel efficiency improvement over a 6x4 tractor are 

2.0%. The results of the other ten sets of test data collected are summarized below, after a general 

description of the common test procedures used to generate such data.

4.2.1 Various Test Procedures

This section briefly describes the various full-vehicle test protocols available for determining the fuel 

efficiency improvement offered by a particular technology. These are not exhaustive descriptions, and 

readers should reference the associated Society of Automotive Engineers (SAE) and or Technology and 

Maintenance Council (TMC) practices for a full understanding of the value and appropriate use of each.

 SAE J1321 / TMC Type II – These tests are used to compare the fuel consumption of a tractor 

with and without a particular fuel-efficiency technology provided that the technology in 

question can be retrofitted onto an existing tractor. In these tests the same tractor and trailer is 

tested with and without the device, generally on a track, though the test can also be done on a 

public road for approximately 50 miles. The tests’ protocols also call for a control truck to be

run without the technology, to allow for the calculation of a ratio between the two trucks when 

run without the technology and ensure consistency of the test. Specifically-designed small fuel 

tanks are used to power the trucks in these tests, and the weight of the fuel consumed, as

determined by removing the tanks and weighing the fuel off the truck, gives the measure of fuel 

efficiency. Multiple valid tests are required for statistical valid results. 

 SAE J1526 / TMC Type III – These tests are very similar to the SAE J1321 / TMC Type II, but are

used to conduct an ‘A-to-B’ vehicle test, in order to evaluate a technology that is difficult or 

impossible to retrofit onto an existing vehicle. In this test there are two test vehicles, one with 

and one without the technology installed, compared to a test and a control vehicle. Most all of 

the other testing protocol details are the same as the SAE J1321 / TMC Type II tests. Although 

much care is taken to ensure the two trucks are identical in specifications for the SAE J1526 / 

TMC Type III tests, component manufacturing variations can affect the results, by introducing

additional variation between the test trucks, such as different powertrains or aerodynamics. 

 SAE J1263 Coast Down – This test is used to evaluate aerodynamic or tire rolling resistance 

features. The drag or rolling resistance can be compared between a vehicle with and without a 

given device by allowing that vehicle to coast to a stop from a consistent speed. The longer the 

distance travelled, over a very flat surface with minimal wind, the more efficient the truck, and 

therefore the more successful the technology was at reducing drag or rolling resistance. 

 TMC Type IV RP1109 – This test was developed by the TMC primarily for use by fleets. The test 

calls for data collection over segments of approximately 400 miles, and can be run while hauling 

real freight, that is, over the course of relatively normal operations. A baseline (control) truck

and a test truck (one with a technology installed) are compared over the route by measuring and 

recording the miles driven and the fuel consumed via “topping off” the normal fuel tanks on the 

truck. This test’s protocol includes criteria regarding the allowable difference in weight between 

the two vehicles, as well as other requirements.
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 In-Use Tests or Freight Mile Tests – No single specific common protocol is used by fleets when 

they compare the fuel consumption of their vehicles with and without fuel efficiency 

technologies. It is common for fleets to procure and compare tractors and/or trailers with and

without the technology by recording fuel usage via telematics or using other methods such as 

fuel card purchases versus miles driven. Generally, these calculations compare sets of trucks 

driving dedicated routes over certain periods of time, or via higher statistical numbers of 

vehicles over for-hire random routes. Each fleet tends to create its own particular methodology 

for capturing and interpreting this data, and to stick with it over time. Though the tests are 

different, certain factors are key to ensuring that a fleet’s protocol is repeatable and the data 

viable, including limiting the difference in the equipment to only the feature under test, 

recording data from many miles run over many months, and using consistent measurement 

methods.

These tests are used by fleets, technology providers, tractor and trailer equipment builders, and others 

to predict the fuel economy of various technologies. Of course, each test has its limitations, and its 

findings may be limited to the particular testing conditions or situations. For instance, a Type II test run 

at 65 mph will predict the performance difference of a technology at 65 mph with limited start and stop 

traffic. A fleet may have a duty cycle that is similar but not identical to this situation – maybe their

average speed is only 55 mph – and this will affect their experience with the technology. Also, the 

entities conducting the tests will sometimes “modify” them and use slightly different protocols, making 

it difficult for others to interpret the top-level results of any such test.

4.2.2 Industry Tests Provided to Trucking Efficiency for this Confidence Report

This section details the different tests that various industry actors, namely OEMs and fleets, have 

recently conducted on 6x2s, and the individual findings of each of those tests, which have been shared 

with the Trucking Efficiency team for the purposes of this Confidence Report. The findings of all of these 

tests are summarized and discussed in the following chapter.

4.2.2.1 OEM #1 –Volvo Global Truck Technology

The first OEM surveyed by this Confidence Report, Volvo, shared with the study team the results of a 

modified Type II (~50 miles) on-highway test that they had privately conducted. Volvo’s test used a

VNL670 control and test vehicle, both equipped with the D13 engines running at US EPA07 emissions 

level. On the test vehicle, the rear tandem was first run as a Meritor 6x4 and then later reconfigured as 

a Meritor 6x2 tandem, maintaining the same gear ratio and tire combinations. The control vehicle 

maintained the 6x4 configuration throughout. The test vehicle was also equipped with Michelin’s WBS 
Confidence Report on 6x2 Axles

1/13/2014 Page 23

XDA Energy tires, a rear axle ratio of 2.64:1 and the Volvo I-shift transmission. The tests were run at a 

GCW of ~70,000 lbs. with the cruise control set at 64 mph which yielded an engine speed of 1138 RPMs 

in top gear. The test route had a total elevation change of less than 200 feet with a maximum grade of 

less than 4%, and with 89% of the route under a 1% grade. Testing was conducted with a very accurate 

positive-displacement-flow meter, plumbed in such a way that the return fuel flow from the engine was 

taken into account. Volvo found that the 6x2 configuration achieved a 1.6% improvement in fuel 

economy over the 6x4. 

4.2.2.2 OEM #2 – Daimler Trucks North America

The second OEM to share data with the study team of this Confidence Report was Daimler Trucks North 

America (DTNA). DTNA ran a 

modified Type IV test (~400 

miles) with two Freightliner 

Cascadia vehicles. The vehicles

were equipped with the Detroit 

Diesel 15L engine, DT12 

transmission, and Michelin XDA 

Energy dual tires on the drive 

axles. The 6x4 vehicle was also 

equipped with Detroit’s DA-RT-40 

tandem rear axles housing a 

2.53:1 axle ratio and the 

combination was loaded to a GCW of 73,600 pounds. For the vehicle with the 6x2 configuration, the 

rear-rear axle was moved to forward-rear position, and a Meritor 6x2 tag axle was used in the rear-rear 

position. It should be noted that this reconfiguration was done solely for the purpose of fuel economy 

testing, and that splitting a tandem drive axle set is not recommended for on-road use without first 

consulting the OEM or axle manufacturer. 

The DTNA test route was over 400 miles with a maximum grade of +/-5%. The 58 mph cruise speed was 

pre-programed into the engine controller to remove driver error and yielded 1280 rpm engine speed. 

Figure 9: Volvo's Test Vehicles

Figure 10: Daimler’s Freightliner 6x4/6x2 Test Vehicle
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DTNA’s test found that the 6x2 configuration achieved a 2.2 +/- 0.3% reduction in fuel consumption 

when compared to the 6x4 axle package. DTNA also did an analysis of the daily average torque between 

the test 6x2 and the baseline 6x4, and found a 2.0% reduction in engine torque, which is similar to the 

measured data. Finally, DTNA also ran a simulation of the tested configuration and achieved a 2.2% 

reduction, which correlates nicely with the actual data from the road test.

4.2.2.3 Fleet #1 – Con-way Truckload

Con-way Truckload is a carrier that has been an early adopter of the 6x2 technology, to the point that 

the majority of their fleet is now running with 6x2s. Con-way runs several different brands of tractors 

and they do keep a few 6x4s in their fleet for comparison. They shared with the Confidence Report 

study team their 12-month rolling data from November of 2012, which contained fuel economy data for 

approximately 2,394 of the tractors in the fleet. Their fuel economy is calculated from the actual fuel 

consumed and miles traveled of those trucks (a “freight miles” test). This study has considered a subset 

of their fleet, comprised of 1,283 of their Kenworth tractors, in order to compare directly between the

1,224 of them that have a 6x2 configuration and the 59 of them that are configured as 6x4s. One caveat 

is that Con-way’s “6x2 Package” also includes a direct drive transmission and wide-based single (WBS) 

tires, neither of which are included on their 6x4 trucks. The remainder of the specifications of their 6x2 

and 6x4 Kenworth tractors are very similar. 

The Con-way 6x2 package yielded a total 6.7% improvement over their 6x4 baseline vehicles across their

12 months’ of data. The Confidence Report study team then consulted with several industry experts on 

the improvement contribution from the direct drive transmission and WBS tires. The consensus was that 

approximately 1.5% of the improvement was from the DD transmission and another 2% was due to the 

WBS tires. Therefore, the 6x2 axle configuration by itself netted a 3.2% improvement.

4.2.2.4 Fleet #2 – Fleet XXX

Trucking Efficiency also received 12 months’ of data from a major fleet who understands the value to 

the industry of sharing data, but wishes to remain anonymous in this Confidence Report. The number of 

trucks in the dataset of their freight miles testing varied as units were removed from service and new 

units were added over the course of the year. In the end, some period of data was available for a total 

of 45 6x4s and 9 6x2s (with the lowest periods of data available for 11 6x4s and 7 6x2s). The trucks’

average monthly mileage was 10,675 (6x4s) and 7,300 (6x2s). This test group was comprised entirely of 

2010 Freightliner Columbia tractors with mid-roof cabs, running in random areas around the country.

The 12 month study ultimately found that 6x2s conferred a 3.9% fuel economy improvement over the 

6x4 configuration.

4.2.2.5 Fleet #3 – UPS 

UPS is another fleet that provided the results of a 12 month study. Their freight miles test was a little 

unique in that two identically equipped Mack Pinnacle trucks with the Mack 13L engine followed each 

other from Charlotte, NC to Atlanta, GA twice a day for a year, accumulating approximately 87,000 miles 

on each vehicle. The only difference between the trucks was that one was equipped with the 6x2 
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configuration and the other was a 6x4. The 6x2 enjoyed a 4.6% improvement in fuel efficiency over the 

6x4 during the 12 months of operation.

4.2.2.6 Fleet #4 – Nussbaum 

The fleet Nussbaum provided a one-month snapshot of their fleet mileage from the month of April 2013. 

Data was made available from their fleet of Freightliners and Volvos, and the Confidence Report study 

team was able to compare between 6x2 and 6x4 Volvos, as the entire group of 113 Freightliners were all 

equipped with 6x2s, with no similarly specified 6x4 trucks. All of the comparison Volvo tractors were 

equipped with modern SCR engines and averaged slightly more than 10,000 miles per month. There 

were four 6x4 Volvos that had overdrive transmissions with WBS tires and ten 6x2 Volvos that had direct 

drive (DD) transmissions and WBS tires. Test data from transmission OEMs suggests that a direct drive 

transmission improves fuel economy by approximately 1.5% versus an overdrive (OD) transmission. In 

the Nussbaum data, the 6x2 DD Volvos enjoyed a 5.48% improvement over the 6x4 OD Volvos.

Subtracting 1.5% for the DD transmissions nets a 3.98% improvement that can be attributed to the 6x2 

configuration.

4.2.2.7 Fleet #5 – Fleet YYY 

Another major fleet willing to share data while remaining anonymous provided the Confidence Report 

study team with four months of freight miles testing data (January through April of 2013) that included

55 tractors with 6x2 configurations and 312 6x4s, and running both Truckload and Less-than-Truckload 

routes. The tractors were both day cab and sleeper International ProStar+ models with MaxxForce 13L

engines, direct drive transmissions, and a 2.47 gear ratio, and overall they averaged about 8,000 miles 

per month. The trucks’ fuel-usage data was all pulled off of their Engine Control Modules (ECMs) and 

reported through the telematics tool, PeopleNET, and it included all divisions/routes of this fleet. The 

fleet’s results showed an average (day cabs and sleepers) of a 1.94% fuel-efficiency improvement for the 

6x2 configuration. The sleepers were found to perform a bit better than the day cabs, enjoying a 2.54% 

improvement versus the day cabs’ 1.53% improvement.

4.2.3 NACFE-Conducted Tests

NACFE was fortunate enough to partner with PIT, Performance Innovation Transport, a group within 

FPInnovations in Quebec, and conduct tests for this Trucking Efficiency Confidence Report as part of 

PIT’s semi-annual fuel economy testing sessions. Three separate tests that compared the 6x2 to 6x4 

configurations were completed. Results are show in Appendix A: PIT Test Report.

The first two tests run were SAE J1321 / TMC Type II involved two 2012 Kenworth T660s from a 

cooperative Canadian-based fleet. These trucks were Canadian spec’ed and ran the larger Dana Spicer 

D40-170P tandem axles with a 3.21 gear ratio, which is essentially a de-rated 46k tandem axle set. The

tests both involved running the two Kenworth trucks in their stock 6x4 configuration to achieve a 

baseline fuel economy metric. The two trucks were then converted into the 6x2 configuration in two 

different manners, providing the two different sets of test data. The first truck (Test 1) had the rear-rear 

axle housing emptied of its differential gear set and axle shafts, and the interaxle drive shafts removed. 

The forward-rear axle had the interaxle differential removed and the gear set changed, but retained the 
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3.21 ratio. The second truck (Test 2) swapped the axle positions. The rear-rear axle moved to the 

forward location and became the single drive axle for the truck without modification. The forward-rear 

axle was moved rearward and was emptied of all its driveline components - gears, differentials and axle 

shafts. This modification also requires relocating the frame side brackets for the transverse torque rods 

for both axles. 

Test 1 resulted in a gross 2.6% 

improvement in fuel economy while Test 2 

yielded a 3.5% improvement. 

The larger axles used in these tests are not 

typically specified in North American linehaul trucks. After several discussions with 

the axle manufacturers on how these larger 

axles would perform when compared to a 

standard 40k tandem, it was determined 

that a 0.7% downward correction factor should be applied, as the larger, 46k, lower-volume tandem 

axles are not as efficient or refined as the higher-volume 40k tandems. As such, removal of one drive 

axle from the less-efficient 46k tandem will see a larger increase in fuel economy improvement. This 

correction to a 40k tandem nets a 1.9% improvement for the 6x2 configuration of Test 1 and a 2.8% 

improvement for the 6x2 configuration of Test 2.

NACFE’s third test involved two similarly specified Volvo VNL 

670s acquired from a dealer lot in Quebec; one a 6x4 (white 

cab) with a Meritor MT40-14X4C tandem, and the other a 6x2 

(red cab) with Meritor’s RS23-160 single drive axle and a Volvo 

non-driving tag axle. This test was conducted according to SAE 

J1526 / TMC Type III, which is appropriate for an A/B 

comparison of two trucks. The 6x2 Volvo achieved a gross 

improvement of 3.3% over the 6x4 Volvo. 

In addition to the drive axle configuration differences, the other notable variance between the two 

trucks was the lack of a few aero devices on the 6x4 model and a slightly larger trailer gap. Moreover, 

the 6x4 had a slightly different aero bumper and was not equipped with chassis shirts, but the undercab, frame-mounted components were tightly packaged with minimal gaps between them. These 

differences were discussed with three industry aerodynamics experts and the team referenced PIT’s 

2007 Energotest on chassis skirts and determined a 1% correction factor for these aerodynamics affects. 

Thus, the net improvement for the 6x2 was 2.3% when corrected against the differences between that 

truck and the 6x4.

Figure 12: NACFE 6x4 Test Vehicle

Figure 11: NACFE's 6x2 Test Vehicle
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4.3 Test Results: Fuel Economy Improvements

The three types of data sets: OEM, fleet, and NACFE, all correlated well with one another. The two OEM 

test sets returned the most conservative results, with an average of a 1.9% improvement in fuel 

economy. The three NACFE/PIT data sets were all 50-mile runs on a highly controlled closed course, and

yielded an average improvement of 2.3%. The data sets from the five different fleets included runs of 

anywhere from 8,000 miles to over 100,000 miles, all conducted in the course of actual freight hauling, 

and with a wide difference in the type and number of trucks tested. The data from the tests of UPS and 

Fleet XXX considered the smallest number of trucks, and therefore potentially a greater effect from 

things like driver influence, weather, etc. The fleet data overall showed an average 3.5% improvement

from installing a 6x2 over a 6x4. It is clear from all of these tests that this technology does deliver a 

substantial improvement in fuel economy, and will significantly reduce a truck’s operating cost for the 

year. 

Table 1 below summarizes the results from the 10 sources of data considered in this 6x2 Confidence 

Report, while Figure 13 provides a graphical representation of the fuel economy results.

Table 1: Fuel Economy Test Results
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Figure 13: Graph of Fuel Economy Test Results
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5 Other Benefits of 6x2s

Along with saving money by improving the fuel economy of a vehicle, 6x2 configurations, whether in a 

package or installed singularly, reduce a vehicle’s weight, cost, and the amount of required 

maintenance, as well as improve its stability.

5.1 Weight

6x2 configurations offer a significant weight reduction for the chassis, as a tag axle itself weighs less than 

a driven axle, plus the full configuration involves the removal of the inter-axle differential and associated 

inter-axle driveline components, as well as one differential gear set. A small amount of weight is added 

for additional features such as load shifting and anti-spin measures – allowing an overall reduction of 

400 to 450 pounds. End users should consult with their truck OEM to determine the proper tandem axle 

suspension system to complement the 6x2 axle configuration and that it is rated to handle the driving 

torque passing through a single axle. Depending on the OEM, the properly rated and configured 6x2 

suspension may negate a small portion of the 6x2 axle weight savings. Many fleets are utilizing this 

weight savings to help offset the increased truck chassis weights that have resulted from the addition of 

new safety and driver amenity options, the last decade of emission controls programs, and the adoption 

of other fuel-efficient but weight-adding features such as aerodynamic fairings.

Reducing weight purely for a fuel economy improvement yields only a small benefit, as a 1,000 lb. 

weight reduction will yield only an approximately 0.8% improvement in fuel economy. A weightreduction feature like the 6x2 configuration is often pursued as an enabler for other fuel-saving but 

weight-adding features like an Auxiliary Power Unit for anti-idling, or full tractor and/or trailer skirting.

Other weight sensitive applications like bulk haul and tankers take advantage of this feature to increase 

their cargo capacity and value weight saving technologies more. WBS tires can be added to the 6x2 

package for approximately an additional 400 lbs. of savings.

5.2 Cost

There is currently approximately a $1,000 to $2,000 upcharge for a new tractor specified with a 6x2 

axle. However, in discussions with the industry, the study team found that the OEMs and 6x2 

suppliers/manufacturers are in wide-agreement that, within a few years, 6x2 configurations will be 

approximately cost-neutral when compared to a similarly specified 6x4. 

The explanation for the current cost differential is that a 6x2 system requires a larger drive gear set, as 

all of the engine torque and engine braking are handled by only one gear set, rather than two as in a 

6x4. Moreover, as will be described in the section below on traction, additional load-shifting electronics 

are recommended for tractors being ordered with 6x2 axles, in order to ensure adequate traction 

performance. Along with these electronics, the 6x2 configuration requires higher-cost 6-channel (6S6M)

ABS/Traction Control systems. Finally, several of the tag axles available for 6x2 systems are custom units 
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which have not yet achieved the high production volumes necessary for lower cost. However, once 

production volume grows, these higher-cost aspects of the systems will be offset by the 6x2’s lack of 

inter-axle prop shafts, inter-axle differential, rear-rear gear set and rear-rear axle shafts – allowing cost 

parity with 6x4s to be achieved.

5.3 Maintenance

The fact that the 6x2 configuration 

allows for a reduction in the total 

number of driveline components 

should lead to an overall reduction in 

maintenance costs and labor. For 

example, the fact that a 6x2 lacks an 

interaxle drive shaft should eliminate 

most driveline vibration complaints,

and it also means that fewer U-joint 

lubrication procedures are needed.

The single drive axle gear set of a 6x2 

reduces the overall differential fluid 

fill at change intervals. Additionally, 

when the tag axle of the 6x2 is in the 

rear most position, the process of stacking tractors for delivery (see Figure 14) is simplified, as there is

no need to remove any axle shafts for transport. The need to remove and then reinstall the rear-rear 

axles on a 6x4 for delivery often leads to warranty issues from axle leakage.

Although there may be a slight increase in maintenance due to the additional 6x2 electronics, the 

reduction in mechanical components and associated maintenance should result in an estimated overall 

savings of $100 per year, available by virture of operating a 6x2 versus a 6x4.

5.4 Stability Improvement 

An often overlooked benefit of adopting a 6x2 configuration is that it might give a truck improved 

stability and control when operating in slippery or poor road conditions. When a 6x4 loses traction, all 

four of its drive tires may potentially spin up. All of the available traction of a spinning tire is consumed

by the spin, so the tire will not be able to provide the lateral forces necessary to keep the vehicle on the 

desired path. Moreover, without any lateral forces at the tandem tire sites, a jack-knife situation can

easily occur. But, with the 6x2 configuration, the tag axle cannot spin up by virtue of not being powered 

by the engine. Thus it will always have some level of lateral forces available to keep the vehicle stable

on its desired path, greatly reducing the potential for a jack-knife event initiated by wheel spin. One 

fleet manager described the tag axle in a 6x2 as a “rudder” that keeps the tractor pointed straight 

ahead.

Figure 14: Piggy-back Delivery of Tractors
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Some fleets have reported a reduction in tow bills or accidents thanks to 6x2s, however their data 

collection systems make it difficult to isolate the 6x2-prevented jack-knife events from all other 

incidents. But given that the cost of a rollover event can exceed $300k in cleanup and damages, 

preventing just one incident has tremendous payback. 

NACFE did not conduct any testing or obtain significant industry data on this particular item. Additional 

investigation aimed at validating the stability improvement of a 6x2 setup versus a 6x4 should be 

conducted.
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6 Adverse Consequences / Challenges 

The following section of this report will detail the various adverse consequences or challenges of using a 

6x2 configuration on over-the-road tractors. 

6.1 Traction

By far the largest concern voiced in the industry about the 6x2 configuration regards a loss of traction or 

tractive effort. Many drivers equate the 6x4’s performance to that of a 4-wheel drive pickup truck, and 

a 6x2 to a 2-wheel drive pickup. But this analogy does not completely hold, given the loads carried by a 

commercial truck compared to a personal vehicle. The normal or vertical force on a commercial tire can 

be as high as 20,000 lbs. on the tire’s contact patch, for either dual or WBS drive tires. Depending on the 

road conditions, a significant percentage of that normal force becomes the available tractive effort to 

move the vehicle forward – 2-wheel drive pickup trucks will never achieve that sort of weight-based 

traction. 

Moreover, many of the conditions where a loss of traction might be noticed with a 6x2 system are likely 

situations a 6x4 truck should avoid or would otherwise have limited traction in as well. Deep snow, deep 

loose gravel, heavy ice, and even parking lots full of pot holes can inhibit traction enough to stop most 

6x4 vehicles. When compared with the 6x4, some of the traction shortcomings of the 6x2 can be 

mitigated with the use of load shifting technologies to increase the weight on the drive axle at low 

speeds.

Unfortunately, the earliest of these ‘load shifting’ systems that were offered along with 6x2 axle 

configurations involved a manual dump valve on the tag axle. The driver could engage the dump valve 

at a low speed, releasing the air from the tag axle air bags and thereby increasing the load on the 

forward-rear drive axle. The tag axle air springs were then re-pressurized once the driver released the 

dump valve, or when the vehicle exceeded speeds of approximately 25 mph. However these systems 

were clunky and required a high-degree of interaction from drivers, making a 6x2 system unappealing to 

some potential adopters.

More recently, these manual dump valves have been much improved. Both tractor OEMs and various 

6x2 component and subsystem manufacturers have developed automated electronic load shift systems 

that are integrated into the truck’s electronic systems and work in conjunction with the electronic 

traction control systems already on the trucks. In these new systems, the trucks’ ABS/Traction Control 

systems sense any drive axle wheel slip and request load transfer from the tag to the drive axle,

engaging the differential lock automatically. As these systems release air from the tag axle springs they 

increase the pressure in the drive axle springs, in order to increase the load on the drive axle while 

maintaining the truck’s ride height. Once the low-traction event is over, the systems automatically

return the air springs to their proper, balanced settings, all without any interaction or input from the 

driver. Some 6x2 load shifting systems carry a slight offset at all times, with a bit more load on the drive 
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axle than on the tag axle, to minimize the number of low-traction events. This offset never exceeds the 

drive axle rated loading and will not allow the tag axle to become completely unloaded. Several of the 

OEMs are now offering proprietary automatic load shifting systems integrated into the vehicle’s 

electronics, while others are providing other manufacturer’s systems like the eTrac from Bendix, or the 

Meritor WABCO ECAS system.

Many fleets that use 6x2s already report that their drivers easily learn their 6x2 systems’ limitations and 

simply try not to drive in really foul weather. By not ‘venturing out into the storm,’ these drivers ensure 

they are never at risk for being stuck or involved in an accident. Although mentioned by most fleets as a 

concern, none of the fleets in our study reported significant issues with 6x2 traction, and many felt that 

their tow bills had even dropped slightly since their introduction of 6x2s. Finally, it must be noted that 

the majority of trucks in Europe are configured as 6x2s or 4x2s, and that these trucks are not only used 

in line haul applications but also in severe service applications such as garbage trucks, dump trucks, and 

snow plows. That is, the European market has been enjoying the efficiency gains of the 6x2 

configuration for years already, and has effectively addressed the traction concerns.

To bolster the finding of this Confidence Report that 

concerns over traction hindering adoption of 6x2 

systems, NACFE conducted a traction test with the 6x4 

and 6x2 test vehicles used in the fuel economy testing 

conducted with PIT. A competition tractor-pulling sled

was acquired and special hitches fabricated to allow 

the tractors to pull the sled. The test was staged on a 

gravel perimeter road at the proving grounds and the 

tractors individually pulled the sled. Similar to 

agricultural tractor pulls at the local county fair, the 

sled weight translates up the sled towards the tractor, 

placing more and more weight on a skid being dragged on 

the gravel road. The test run stops once the tractor loses

traction, with its wheels spinning and unable to move 

forward any further. Each truck made one practice run to 

allow the driver to become familiar with the vehicle, and 

then the distance traveled was recorded for three

consecutive test runs. The average distance traveled for 

each set of three runs was calculated, and comparisons 

were made between the 6x2 and 6x4 vehicles. A video of 

this testing is available at www.NACFE.org.

Table 2: Traction Testing Results contains the results from the traction testing. The test found that the

Kenworth 6x2 vehicle, with its tandem air suspension in normal operation, traveled about 11% less than 

the standard 6x4 unit. But the 6x2 truck was also equipped with a manual dump valve for the tag axle.

Figure 15: Sled Pull Traction Test

Figure 16: Hitches for Tractor Sled Pull
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When this system was utilized and the weight was shifted, the 6x2 traveled an additional 15 feet, ending 

up only 5.4% off the distance the 6x4 was able to travel. The Volvo trucks had similar results, with about 

a 13% reduction in distance traveled for the 6x2 when compared to the 6x4. However, the 6x2 Volvo 

truck was not equipped with a load shifting system, and the impact of such a system could not be 

measured. 

 

Although this simple tractor pull test does not give comprehensive or conclusive data on the tractive 

effort of a 6x2, it is an indication of the capabilities of the system, and it does confirm both 6x2 

manufacture claims and the experience reported from fleets using 6x2s already in Europe and North 

America. The tag axle dump test on the Kenworth bolstered the conclusion that a manual dump valve or 

automated load shifting system allows a 6x2 to achieve improved tractive performance. The 

manufacturers of axle automated control systems have additional information and videos on 6x2 

traction. Appendix A: PIT Test Report contains the test report on 6x2 traction. 

6.2 Tire Wear

The second-most mentioned concern heard by the study team surrounded the tire wear on the single 

drive axle of a 6x2 system, given that, in a 6x2 configuration, all of the driving torque and engine braking 

torque passes through the single set of tires on the single drive axle, those tires will experience 

increased wear. Some in the industry assume that the driven tires of a 6x2 will wear twice as fast as the 

tires on a 6x4, since only half as many tires are providing thrust. But data from tire manufacturers and 

several fleets indicates that the useable tire life on a 6x2 drive axle is actually about one third that of the 

6x4 drive tires.

Tractor Pull Traction Testings

Average 

Distance (ft)

% Change 

from 6x4

Average 

Distance (ft)

% Change 

from 6x4

Kenworth 6x4 256 Volvo 6x4 233

Kenworth 6x2 - Suspension Aired 227 -11.3% Volvo 6x2 201 -13.5%

Kenworth 6x2 - Tag Axle Dumped 242 -5.4%

Table 2: Traction Testing Results

Figure 17: Sled Pull Machine Figure 18: Custom Hitch Arm
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Fortunately, there are several options made available by a 6x2 system that can offset the increased cost 

of this faster drive tire wear. A less expensive trailer tire can be used on the tag axle, and given that 

these tires also have a lower rolling resistance than heavy lug drive tires their use will save on fuel costs 

as well. Retread tires can also be used in the tag location, further reducing overall tire cost. In any case, 

no matter which tires are used, the fuel efficiency gains offered by the 6x2 configuration more than 

offsets the increased tire cost, as demonstrated in the payback calculator that supplements this 

Confidence Report. Depending on the tire type and cost, and the truck specifications, the fuel savings 

are typically 3 to 7 times greater than the increased tire costs of a 6x2. 

Any significant analysis on the tire wear issue was not conducted as part of this study. Some mentioned 

increased tire cost ranging from $400 to as much as $1,000. Those considering 6x2 purchase, should 

discuss this issue with their tire manufacturers and other fleets. A rough calculation based on one fleet’s 

observations with the WBS tires and the 6x2 configuration was created and is presented here. Table 3

shows the cost projection data of that fleet on the increased drive tire wear experienced during a typical 

over-the-road tractor duty cycle. The top portion of the table provides details on the different individual 

tire’s projected performance and cost for a line haul fleet. The life of the tire, or pull point is based on 

4/32nds of remaining tread depth. For the XDN2 tire starting with 27/32nds of tread and pulled at 

4/32nds, it has 23/32nds of usable tread. Tire cost per mile is calculated by taking the retail tire cost 

divided by the Mileage Pull Point for either the 6x4 or 6x2 configuration.

The lower half of Table 3 contains the tandem tire cost per tractor and excludes the steer axle and trailer 

tire cost as those remain unchanged in this study. In Option #1, life cycle cost for the 6x4 drive axles 

with four XDN2 tires are compared to the 6x2 with two XDN2 tires on drive axle and two XTA tires on the 

tag axle. In this rough example, running the 6x2 has an increased cost of just under one cent per mile 

and is under a half cent if you are comparing the XDNA Energy tires. If you assume the average line haul 

tractor in the fleet is running 100,000 miles per year, then the annual increase in tire cost per 6x2 tractor 

is just under $500 or $1000 depending on your comparison tire. 

Table 3: Tire Cost Analysis for Typical 6x4 vs. 6x2
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Specifying a 6x2 tractor with a traction control system and/or automatic load transfer system will further 

mitigate excessive drive tire wear, as these systems can respond immediately to wheel slip and thereby

minimize unnecessary spin up of the drive tires. Also, fleets can adopt higher-wear though slightly 

higher rolling resistance tires and tire manufacturers, recognizing the potential of 6x2s and expanded 

use of 4x2s are developing improved tires for these applications. Finally, driver education programs on 

the use and limitations of the 6x2 configuration can also help to minimize the drive tire wear.

6.3 Resale Value

This study finds that the effects a 6x2 configuration can have on the resale or residual value of a used 

truck at time of sale vary widely. First of all, some users, particularly private fleets, will operate their 

trucks to or close to the end of their useful life, so resale value will not even be a factor in their purchase

decision. Other fleets who have embraced 6x2 technology and are achieving higher-than-average miles 

per gallon told the study team that they have buyers available to purchase their used rigs. But still other

users reported their belief that 6x2s will sell for upwards of $4,000 less than a comparable 6x4 tractor. 

In Tom Berg’s May 2013 Heavy Duty Trucking article “Is a Fuel- and Weight-Saving 6x2 for You?” he 

states that “the Truck Blue Book numbers for a late-model 6x2 tractors being worth $11,000 less than a 

6x4, 2007-09 models at $5,500 less, yet the 2004-06 6x2 models were listed at $700 more. Go figure.” 

However, the study team’s interviews with fleets, OEMs, and manufacturers, found that many shared 

the belief that the resale value of 6x2s is rising as the technology is better understood and the industry 

becomes better educated on its limitations. In particular, the automated traction control and load 

transfer systems address many of those concerns, so interviewees reported that, as those systems 

become more prevalent in 6x2 tractor specifications, the resale value of 6x2s should match that of 6x4s. 

In fact, as awareness of the lower operating costs and improved fuel efficiencies of 6x2s grows, a 6x2

truck may eventually garner a premium over a 6x4. Many owner-operators are seeing the advantage of 

6x2s in trucking media and therefore are actively seeking 6x2s for their next used truck purchases. 

Finally, the industry reported to the study their belief that even if there is a resale penalty on the 6x2 

configuration today, when a fleet might be considering whether or not to purchase their new trucks with 

6x2s, that resale penalty may be gone by the time those fleets are ready to take that vehicle out of 

service to be sold. But a handful of other fleets cited this lower resale value as the reason for their 

choosing not to specify their next tractor purchases with 6x2s. 

6.4 Driver Acceptance

Many people are adverse to change and professional truck drivers are, perhaps, especially so, when it 

comes to their workplace. The 6x2 is different – it’s a change from what most drivers are used to 

operating. Driver training on the proper operation of a 6x2-equipped vehicle is required when adopting 

them, just like with any major new technology. However, the newer traction control and automated 

load transfer systems remove a substantial amount of onus for proper operation of a 6x2 truck from the 

driver, and can make the transition from 6x4 to 6x2 almost seamless. For this and other reasons, this 
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Confidence Report highly recommends those systems to enhance the drivers’ experiences with 6x2s, as 

well as to reduce wear and tear on the drivetrain and drive tires and to ensure good traction.

Many fleets have made a commitment to 6x2 technology and moved quickly to transition their entire 

fleet to the 6x2 configuration. Those fleets report that with an appropriate driver education system, 

they have had few driver issues or complaints, and noted little-to-no impact on their overall operations. 
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7 Original Equipment Manufacturers’ Perspectives on System 

Integration

The Confidence Report study team conducted interviews on 6x2s with four of the six tractor OEMs in the 

sector. The following are the key points from these discussions:

 6x2 axles have been touted as an option for improving tractor fuel efficiency for decades, but 

the adverse consequences of lost traction and increased tire wear, whether perceived or real, 

have been sufficient to prevent their widespread adoption. 

o European tractors are predominately 4x2 configurations, but some 6x2s are in use

where loading requires the additional tractor axle.

 By 2010, some early adopter fleets in North America were experimenting and asking for 6x2 

configurations on their new trucks. Given increasing fuel prices, and the fact that a few medium 

and large fleets requested these axles, tractor OEMs began designing and supplying the feature 

on a “by order” basis. 

 By 2013, according to both axle manufacturers and multiple OEMs, the availability of 6x2s has 

grown such that 6x2s from both axle manufacturers are now a data-book offering at 5 of the 6 

OEMs. Also, OEMs report that they are integrating the complementary technologies such as 

automatic load shifting systems into their product offerings this year.

 Production rates of 6x2 axles as a percent of Class 8 production at four of the six tractor makers

is shown in Table 4. These are production rates as of the second half of 2013 and average about 

2.3% of the over-the-road tractor builds.

Table 4: 6x2 Production Rates by OEM
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 Some OEMs are developing their own 6x2 products, including their own non-driving axles and/or 

proprietary electronics to manage traction events.

 All four SuperTruck programs – Peterbilt/Cummins, Daimler, Navistar, and Volvo – utilize 6x2 

axles in their solution set. The SuperTruck program is an industry cost-shared development and 

demonstration project initiated by the US Department of Energy to validate the possibility of 

improving freight efficiency of tractor trailers by 50%.

 Tractor OEMs believe that 6x2 adoption will grow over the next few years. 

o As one General Manger shared, “We don’t know why more fleets, big and small, are not 

buying 6x2s. Our investigation shows that the benefits are clear”. 

 Discussions with major fleets on purchases of 6x2 axles is increasing. Most fleets are considering 

increasing their purchase of 6x2s or at least buying some for testing. 

 A small niche of owner operators who are very active in pursuit of high fuel efficiency, are 

retrofitting their trucks into 6x2s.
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8 Summary of Fleet Perspectives and Experiences with 6x2 

Configuration

The study team also interviewed over a dozen end-user fleets to get their perspectives on 6x2 axles.

Those fleets were qualified into two groups: ‘early-adopting fleets’ who have already purchased 6x2 

axles, and ‘evaluating fleets’ who are interested in 6x2s but have not yet made a decision to pursue 

them. Finally, the study team conducted an additional internet survey of fleets through a Fleet Forum 

provided by Michelin NA.

8.1 Early-Adopting Fleets:

 In the mid-2000s, a few innovative fleets began to investigate 6x2 axles, and were able to 

convince “somewhat resistant” OEMs to help them test and incorporate those axles onto their 

trucks.

 These fleets were motivated by what they saw as an opportunity to use 6x2s to simultaneously 

lower fuel costs, reduce tractor weights, and limit or even reduce other costs.

 These same fleets now compliment the OEMs and axle manufacturers for their integration 

efforts and current product offerings.

 Due to the high level of physical change to a tractor between a 6x4 and a 6x2, most fleets 

believe 6x2s to be best applied onto new tractors, rather than retrofit onto 6x4 trucks.

 The only significant negative of 6x2s in the opinion of the early-adopting fleets was traction. 

Therefore the earliest adopters tended to operate trucks in the extreme southern states of the 

USA.

 After much investigation, these fleets decided to pursue 6x2s anyway, and to make the best use 

of manual dump valve technology available at the time by proactively working with their drivers 

and maintenance technicians. 

 A few of these fleets noted that their tow bills and accident rates for vehicles operating in 

extreme weather conditions are actually lower with 6x2s, contrary to what concerns over 

traction might suggest. 

o “Drivers are not taking the risks to drive in bad weather conditions or off pavement 

where they may get stuck,” stated one fleet manager. 

 Early-adopting fleets chose to go “all-in” and purchase the 6x2 package of 6x2 axles, direct drive 

transmissions, WBS tires, and trailer tires on the non-driven axle. 

o “Change was best made in bunches” said one.
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 Most of the fuel efficiency data available from these fleets was for that package, and these fleets

could not “break out” the performance of each of the features independently. 

 In 2012 and 2013 a few of these fleets purchased trucks with each of the features installed 

individually, allowing for some granularity of performance data. All of the fleets that did so

confirmed that they will continue to specify and buy 6x2 axles on as many of their new trucks as 

possible.

 Tire life is now being studied in detail, and a few of these fleets are switching to a longer-lasting

drive tire in spite of those tires’ higher rolling resistance, for example moving to a Michelin XDN2 

from an XTA wide base single tire. This is a tradeoff between lower tire expenses and slightly 

higher fuel costs, but it appears that even with this tradeoff 6x2 axles will improve fuel efficiency 

substantially enough to justify their continued adoption. 

8.2 Evaluating Fleets

 These fleets report that they have been watching the experiences of the early-adopting fleets

with 6x2 axles, and that, for at least the past two years, they have been considering 6x2 axles for

their specific specs/package, procuring a few and testing them in various duty cycles. All of 

these fleets are calculating the paybacks and weighing all the benefits and adverse 

consequences of 6x2 adoption.

 These fleets report that their need for weight reduction is growing, particularly with the added 

weight inherent in complying with the 2007 and 2010 US EPA emissions regulations, as well as

the adoption of other fuel efficiency technologies such as aerodynamics devices.

 When asked about traction concerns, they stated they are worried about operating 6x2s in 

snowy mountain regions and that their network must have all trucks available to haul freight 

anywhere in the US. These fleets, though, referenced the minimal issues reported by the earlyadopting fleets, as well as their confidence in the automatic load shifting technologies now

available, which they plan to specify on all 6x2 tractors, if they decide to pursue 6x2s at all.

 Tire wear is a concern for these fleets, but given the potential for improved tire life on the tag 

axle, and the sense that the life of the drive axle tires is not so reduced as to be unmanageable, 

tire wear is not preventing their adoption of 6x2s.

o Evaluating fleets reported consulting with other fleets, tire companies, and tractor 

OEMs to make highly-informed decisions on tires. 

 These fleets see the concerns around both driver acceptance and resale value as “selfcorrecting” with higher adoption rates, which will come as more and more end-users see the 

benefits of 6x2s. 
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o But they admitted that driver attraction and retention is one of a fleet’s largest 

challenges, and therefore any specification on a truck that limits the availability of 

drivers for a fleet can be a major issue. 

 These fleets felt that 6x2 costs seem to be coming down with design refinement and scale.

 “We are trying to hit the adoption ‘sweet-spot,’” said one major fleet manager. He continued, 

“As more drivers hear about the small if not unnoticeable difference [between 6x2s and 6x4s], 

and demand for used 6x2 trucks increases, these issues will mitigate.” He shared that his fleet 

believes the ‘tipping point’ for 6x2s is 2014.

 One fleet from this group has already ordered 220 new tractors with 6x2s after a 12 month trial,

and another very large for-hire fleet expects to change their standard tractor specifications to 

6x2s on their 2014 purchases.

8.3 Fleet Forum Internet Survey

Michelin North America offered the use of their Fleet Forum to the study team in order for Trucking 

Efficiency to better understand the attitudes toward 6x2 axles held by both users and non-users of the 

technology. The Michelin Fleet Advisory Council is an online community of 300 plus members ranging 

from owner-operators to fleet managers who oversee power units, class 6-8. The Council is available on 

a 24x7 basis to answer questions, and the forum is also able to capture unsolicited insights that emerge 

from member-initiated conversations.

For purposes of this Confidence Report, the members of the Michelin Fleet Forum were asked to answer 

some questions with respect to the 6x2 technology. This survey was conducted from July 17th to July 

31st, 2013. 51 fleets responded. 

 A quarter of respondents currently use a 6x2 axle configuration on their fleet. The majority of 

those were large fleets.

 Those who do use 6x2s reported that they have experienced fuel savings of 0.5 mpg, and weight 

savings ranging from 300 lbs. to 1,500 lbs.

 The biggest concerns experienced by users are traction, resale value, and driver acceptance.

Tire wear was hardly mentioned.

 Those who have yet to purchase a 6x2 axle configuration do believe that 6x2s offer significant 

fuel and weight savings, but they are not convinced that they will be able to sufficiently 

overcome the challenges of traction and driver acceptance. 

 Of those who do not currently use any 6x2 axle configurations, half say that they are likely to 

purchase a vehicle with a 6x2 system in the near future, due to the fact that they now perceive 

the benefits to outweigh the risks, especially given the existence of systems such as automatic 
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traction control (ATC) and electronically-controlled air suspension (ECAS) that mitigate the risk 

of low traction.

 Fleets who have already bought 6x2s reported that it is too early for them to tell if traction is 

indeed a major challenge that they will face throughout the life of the vehicle, saying:

o “Haven’t taken them through winter in the north yet, but with ECAS we expect good 

results.” 

o “Too early to tell if loss of traction is an issue.” 

o “We get stuck in off road situations more frequently, however, the drivers are learning 

their limitations.”

 Fleets who do not currently use any 6x2 axle configuration reported that they believe 6x2s will 

create lower rolling resistance, allowing the vehicle to save more fuel:

o “Increased fuel mileage by reducing mechanical drag/friction.” 

o “Lower weight, reduced rolling resistance and mechanical resistance resulting in 

increased fuel economy.”

 Fleet managers who have yet to equip any vehicles with 6x2s reported that they do expect that this 

configuration would negatively impact the vehicle’s traction, voicing their concerns around 6x2s:

o “More challenging for drivers to maximize traction in slippery conditions.” 

o “Winter operation in snow country.” 

o “Traction in mountainous areas. That is about the only concern at this point.”

 After considering both the benefits and challenges presented with 6x2 axle configurations, about 

half of respondents who had not yet purchased any 6x2s said that would be likely to purchase a 

vehicle with such a configuration, saying:

o “This isn't really new, there are a few fleets that are running these now, so I am not afraid to 

try it.” 

o “Today's emission-controlled engines are heavier and I will need the weight savings of a 6x2 

to offset some of that increased weight.” 

o “With the new braking systems and ATC we are likely going to try some. We definitely feel 

there is a benefit to them.” 

o “Fuel is our single largest expense. The more we can do to reduce fuel costs the more 

profits we keep.”
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9 Payback Calculator

Along with this Confidence Report, Trucking Efficiency has created a payback calculator that estimates 

the payback, in months, for end users who adopt a 6x2 system on their tractors. End users are asked to 

input a small amount of data into the form, and the calculator uses that data along with information 

gathered by the study team to quantify the benefits and consequences likely to be experience by that 

end user, both in terms of upfront costs and year-over-year costs. This calculation is highly dependent 

on the upcharge or down charge for 6x2s as part of the overall tractor cost, as well as the resale value if 

the fleet is selling the truck before its useful life is completed. Table 5 shows an example of the payback 

calculator in action, assuming that both the upfront cost and the resale value of a 6x2 truck as on par 

with that of a 6x4 truck, as is widely predicted to occur in the next year or so.

Table 5: Payback Calculator
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10 Fleet Implementation Best Practices

After analyzing all the OEM, end-user, and fleet forum inputs, Trucking Efficiency developed this list of

best practices for 6x2 adoption:

 Thoroughly study all available information on 6x2 axles.

o Complete a detailed payback analysis for your particular duty cycle and business model. 

o Consider testing the configuration in your fleet only if it is necessary – a lot of data 

already exists – testing may not be required.

 Consider “going all the way” with quick adoption of 6x2s on all new tractor purchases – plan to 

go to 100% in the next 1-2 years on all new truck purchases.

o Make this your NEW truck. 

 Tell drivers, maintenance technicians; basically everyone that this will be ‘our 

new truck.’

o Make 6x2s the specifications of your driver training trucks immediately.

 Include automatic load shifting systems your specifications for 6x2 trucks, and consider including

direct drive transmissions as well as wide base single tires and/or lower rolling resistance tires, 

with trailer tires on the tag axles.

 Create a resale strategy.

o Communicate with end-user buyers to educate them on your experience with the 6x2 

trucks, including the benefits you have enjoyed and the strategies you have used to limit

any adverse consequences. 

o Consider lengthening your ownership lifecycle plans if necessary, perhaps even by 

changing a few other specifications to allow for longer truck ownership. 

 Collaborate with axle and tractor suppliers to get the best specifications for your needs, and to 

manage the costs for the feature on your new truck purchases. 

o 6x2s could result in a cost penalty for now, but effective management will decrease this 

over time.

 Train drivers on traction solutions including shutting down the trucks in extreme weather 

conditions.

o Plan for some higher tow instances and the associated bills, but be prepared to in fact 

enjoy lower accidents and breakdowns.

 Stay the Course.

o As with many changes, diligence to the change is critical to success.
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11 Perspectives on Future Axle Products

Several technologies and advancements are in the works which will enhance the performance of 6x2s 

and improve on their already high fuel efficiency. Many of these technologies will not be available for 

several years, and therefore did not fit into the scope of this report. 

Fleets should be on the lookout for continued advancements in gear-set designs and lubricants which 

further reduce parasitic losses. There will likely be continued integration of engines and transmissions, 

not only the hardware but also the electronics, which combined will optimize drive cycles for maximum 

fuel efficiency. Engine cruise RPMs will continue to drop until the OEMs find the ‘sweet spot,’ and axle 

manufacturers will need to support these changes with the optimized gear ratios. Finally, predictive 

cruise controls and geo-course plotting systems are also under development in an effort to minimize 

losses from hill climbing and even head winds.

More generally, as the 6x2 concept experiences wider and wider adoption, the market may see some 

variations on the current configurations. There have been some discussions around introducing a 

pusher axle system into North America in place of the currently-available tag axles, and a couple of

owner operators are already experimenting with this setup, though pusher axle systems are a bit more 

complex and not nearly as weight efficient, given the longer driveline components required to reach the 

rear-rear drive axle position. Other discussions surround liftable axles, further reducing the rolling 

resistance by lifting the entire axle off the pavement. Available in pusher or tag, but there hasn’t been a 

lot of interest in these thus far, and the benefits of such a system are unclear.

But the most promising and significant advancements surrounding 6x2 systems will likely involve their

electronics and controls, as the OEMs and suppliers are continuing to work on the automated controls.

Most likely, some truck OEMs will integrate traction and load shifting controls into their truck electronic

systems, in an effort to reduce complexity and limit the number of electronic modules on the vehicle, as 

well as to enhance system performance. System suppliers will also move to integrate brake, traction,

and load-shifting controls into one integrated package, also to reduce cost and complexity while 

enhancing overall system performance. 
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12 Conclusions and Recommendations 

Major conclusions from this study are:

 Early-adopting fleets have paved the way for the next wave of wider adoption.

 Axle manufactures and truck builders have integrated this feature successfully into their 

products, and availability is widespread for fleets to pursue 6x2s with confidence.

 Given all the data in this report, Trucking Efficiency predicts a rather aggressive adoption of 6x2 

axles on new truck production over the next few years, possibly doubling every year. 2% in 

2013, 4% in 2014, 8% in 2015 and 16% in 2016.

 Fuel economy improvements of about 2.5% and paybacks of around 20 months should be 

expected from 6x2s.

 A significant amount of data and test results around fuel efficiency, traction, tire wear and resale 

value is available such that this report’s confidence in 6x2s as a good investment adoption is 

medium to high. 

Figure 19: Trucking Efficiency Confidence Matrix
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This Confidence Report recommends that fleets should:

 Consider rapidly adopting 6x2 axles, given the information contained in this report.

 Use the Best Practices stated above, and stay appraised of any new developments around 6x2s.

This Confidence Report recommends that axle manufacturers and truck builders should:

 Continuously improve the performance, reliability/durability, and cost of 6x2 systems as their 

adoption grows.

 Continuously develop complementary additional features for 6x2 systems.
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13 References

13.1 Media & News Articles

“@MATS: Meritor showcases Euro-style 6x2 rear axle package,” Today’s Trucking, March 31, 2011.

Allen, Charlie, “Meritor Axle Efficiency,” Meritor, Inc., PowerPoint presentation, June, 2012.

“Bendix Air Pressure Transfer System to Address 6x2 Traction Concerns,” Truckinginfo, March, 2012.

Berg, Tom, “Is a fuel-and weight-saving 6x2 for you?,” Truckinginfo, May, 2013.

Berg, Tom, “Meritor Expects 6x2 Acceptance to Grow, Readies Faster Axle Ratios,” Truckinginfo, 

December 6, 2012.

Choike, George, “Spec’ing and maintaining rear drive axles for optimum vehicle performance,” 

ArvinMeritor, 2001.

Dana Holding Corporation Holding Corporation, “SPICER® ECONOTREKTM 6X2 TANDEM,” Press release, 

March 20, 2013.

Dana, “SPICER® ECONOTREKTM 6X2 TANDEM,” PowerPoint presentation, 2013.

Dana Holding Corporation Spicer Drivetrain Products, “Spicer® EconoTrekTM Tandem Axle: Durable 6x2 

design improves fuel efficiency and performance while reducing maintenance,” Brochure, 2013.

Kilcarr, Sean, “Spicer debuts new tag axles and conversion kit,” Fleet Owner, February 10, 2010.

“Light in weight. Heavy on performance. Featuring the FUELiteTM Tandem Axle,” Meritor Inc., Brochure. 

2012.

Lockridge, Deborah, “Trends from Mid-America: 6x2s,” Truckinginfo, March 26, 2012.

Menzies, James, “The 6x2s are coming,” Truck News, December, 2012.

Menzies, James, “Spec’ing tactics evolving with rising equipment costs: US fleet exec American Central 

Transport gains 4/10ths of a mpg using 6x2 tractors,” Truck News, September, 2012.

“NACFE conducts extensive benchmarking study on fleet fuel efficiency,” Truck News, February 21, 2012.

“New Dana Spicer convertible tag axles offered,” Fleet Equipment, 2012.

North American Council for Freight Efficiency, “2013 Annual Fleet Fuel Efficiency Benchmark Study,” 

March 10, 2013.
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North American Council for Freight Efficiency, “Executive Report – 6x2 (Dead Axle) Tractors,” November, 

2010.

“Roadranger: The new Spicer tag axle,” Dana Spicer Drivetrain Products; Eaton, 2009.

SAE International Surface Vehicle Recommended Practice, “Joint TMC/SAE Fuel Consumption Test 

Procedure – Type II,” SAE J1321 (TMC RP-1102) Type II, Rev. 2012.

SAE International Surface Vehicle Recommended Practice, “Joint TMC/SAE Fuel Consumption In-Service 

Test Procedure Type Iii,” SAE J1526 / TMC Type III, 2011.

SAE International Surface Vehicle Recommended Practice - "Road Load Measurement and 

Dynamometer Simulation Using Coast Down Techniques," SAE J1263, 2010.

Technology & Maintenance Council TMC Type IV RP1109 Type IV Fuel Economy Test Procedure. 

United States Department of Energy : National Energy Technology Laboratory, “Vehicle Systems, 

SuperTruck – Development and Demonstration of a Fuel-Efficient Class 8 Tractor & Trailer”, May 17, 

2012.
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14 Appendix A: PIT Test Report
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