Factors 

Affecting Truck 

Fuel Economy

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Factors

Affecting Truck

Fuel Economy
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Factors

Affecting Truck

Fuel Economy

VEHICLE AND

ENGINE DESIGN

A. Performance Factors

Fuel consumption is a function of

power required at the wheels and overall

engine-accessories-driveline efficiency.

Factors that affect fuel consumption

at steady speeds over level terrain are:

Power Output-Engine-AccessoryDriveline System

1. Basic engine characteristics; fuel 

consumption vs. RPM and BHP.

2. Overall transmission and drive axle

gear ratios.

3. Power train loss; frictional losses in

overall gear reduction system.

4. Power losses due to fan, alternator,

air-conditioning, power steering, and

any other engine-driven accessories.

Power Required - Vehicle and Tires

The horsepower required for a vehicle

to sustain a given speed is a function of

the vehicle’s total drag. The greater the

drag, the more horsepower is required.

The total vehicle drag can be broken

into two main components; aerodynamic

drag and tire drag. Factors affecting

these components are:

Factors

Influencing Drag

Aerodynamic – Vehicle speed

Vehicle Frontal area

Vehicle Shape

Tire – Vehicle Gross Weight

Tire Rolling Resistance

Both aerodynamic drag and tire drag

are influenced by vehicle speed. It is

important, though, to note that speed

has a much greater affect on aerodynamic

drag than on tire drag, Figure 1.

Gains in fuel economy can be made

by either optimizing or reducing some 

of the factors affecting drag.

B. Type of Vehicle

The type of vehicle affects aerodynamic

drag through its size (frontal area) and

shape. The following illustration shows

two tractor-trailer combinations which,

as a result of their shorter height 

(h2 and h3), have smaller frontal areas

than the standard van-type trailer.

Trailer shape has a large impact on the

aerodynamic drag of the tractor-trailer

combination. Some examples of trailers that

have lower aerodynamic drag shapes are:

FIGURE 1

Vehicle Speed

vs.

Aerodynamic Drag and Tire Drag

Vehicle Speed

Drag Force

30 40 50 60 70 80

Tire Drag

Aerodynamic Drag

Where: h1>h2<h3 Frontal Area = FA = (h) x (w) Where: h = Height; w = Width

h1

h2

h3
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Factors

Affecting Truck

Fuel Economy

Drop frame trailers – Less “Open Air”

space under the trailer. This also creates

less airflow disturbance in crosswind

conditions and thereby reduces the

amount of drag.

Sharp Vertical Edge

Rounded Vertical Edge – Maintains

“Attached” airflow along the trailer

sides, which reduces drag.

Airflow

Airflow

Airflow

Airflow
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Factors

Affecting Truck

Fuel Economy

C. Use of Aerodynamic 

Drag Reduction Devices

With van-type trailers, certain add-on

devices are capable of reducing a vehicle’s

aerodynamic drag. These devices help

maintain an “attached” airflow along the

trailer sides. Again, an increase in drag

occurs when the airflow becomes “detached.”

The favorable impact of roof fairings

is maximized when the vehicle is operating

in a “head-on” wind condition as shown

above. The effectiveness of a roof fairing

is reduced when the vehicle encounters a

“crosswind” (yaw wind) condition. Also,

if the trailer height is lower than the top

of the fairing, as in the case of a flat-bed

trailer, the fairing increases drag because

it increases the vehicle’s frontal area. Use

of a “roof shield” is less effective than a

“roof fairing” because it doesn’t channel

the wind at the sides. Therefore, a “roof

fairing” is preferred.

Vertical gap seal devices reduce drag

by preventing the airflow from entering

the “open air” space between the tractor

and trailer. Unlike the roof fairing, the

impact of this device is maximized 

when the vehicle is operating in a 

yaw wind condition.

D. Engine and Driveline

Characteristics

The use of wide torque band low

RPM engines and wide-step top gear

transmissions, combined with proper 

rear axle ratios, leads to fuel economy

improvement when operated in the

speed and RPM ranges recommended 

by engine and vehicle manufacturers.

Note that a change in the overall

diameter of the drive axle tires can 

effectively alter the rear axle ratio and

could adversely affect fuel economy. The

determination whether a drive tire change

produces an increase or decrease in fuel

economy depends on how much and in

which direction engine RPMs are changed.

Side Gap Seal

Vertical Gap Seal

Airflow

Also of importance is the amount of gap

between the back of the tractor cab and

the front of the trailer. The larger the gap,

the greater the disruption to the airflow

and the resulting drag. This becomes

even more important when encountering

crosswind conditions (yaw wind). A rule

of thumb is for every 10'' over a 30'' gap

there is about a 1/10 drop in MPG.

Long Wheelbase Tractor

Yaw Wind Condition

Airflow

Gap

Gap
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Factors

Affecting Truck

Fuel Economy

VEHICLE OPERATION

The effect of tire overall diameter on

fuel consumption can be illustrated using

an engine fuel map, Figure 2. This is an

example of a typical part load brake specific

fuel consumption (BSFC) engine map. It

shows lines of constant BSFC as a function

of engine BHP output (vertical axis) and

engine RPM (horizontal axis).

A smaller diameter drive axle tire

results in an increase in engine cruise

RPMs, from point A to B. At point B the

engine is consuming more fuel for the

same BHP output.

Proper drive train component matching

can provide the most fuel efficient RPM/

ground speed combination to maximize

fuel economy. Engine RPMs can be

determined using the following formula:

Engine RPM=V x TR x AR x (Tire RPM)

60 Where:

V = Vehicle Speed (mph)

TR = Transmission Ratio @

Top Gear (e.g. 1.0 for 

Direct Drive)

AR = Rear Axle Ratio (e.g. 3.70)

Tire RPM = Tire Revs Per Mile 

(obtained from Goodyear’s

Engineering Data Book or 

www.goodyear.com/truck)

Lines Of Constant BSFC

Increasing Fuel Consumption

Engine Speed - RPM

Engine Output - BPH

600 900 1200 1500 1800 1200

360

330

300

270

240

210

180

150

120

90

60

30

A B 

A = Cruise Point @ 0% Grade, 

 80,000 Lbs. GCW

FIGURE 2

A. General

Consider a typical tractor and van

combination operating at 80,000 lb.

gross combination weight and at 55 MPH

on a level highway. No aerodynamic

drag reduction devices are used on

either the tractor or the trailer. Using

bias ply tires in all wheel positions, the

approximate distribution of horsepower

requirements is as follows:

HP

Item Requirement Percent

Aerodynamic Drag 104 40

Tire Roll Resistance 97 38

Driveline Losses 36 14

Engine Accessories 20 8

257 100

In this example, the horsepower

required to overcome bias ply tire rolling

resistance is essentially the same as that

required to counteract aerodynamic drag.

The total horsepower requirement can

be lowered with the use of radial ply tires.

Because radial ply tires have lower

rolling resistance than bias ply tires, 

tire horsepower requirements are lower.

As a result, fuel economy is improved.

And as the proportion of tire horsepower

requirement on a vehicle increases, the

gain in fuel economy due to using radial

truck tires increases. Some examples 

of tire horsepower requirements as a

percentage of total vehicle horsepower

requirements are given in Figure 3.

At lower Gross Combination Weights

(at the same speed), the horsepower

required to overcome the tire rolling

resistance is a smaller portion of the

total brake horsepower required (BHP).

This is also true as speed is increased (at

the same GCW). As the vehicle’s aerodynamics are improved, as in the case of

a tractor pulling a tanker trailer rather

than a van trailer, the BHP required to

overcome aerodynamic drag is reduced.

This has the effect of increasing the

percent contribution of tire rolling

resistance to the total BHP required. In

this case, reducing tire rolling resistance

by switching to radials has a greater

impact on reducing the total BHP

required.

FIGURE 3

Tractor-Trailer Horsepower Requirements 

By Component 

Engine Brake Horsepower Required

400

300

200

100

0

Van Trailer

257

237

179 172

357

334

55 MPH

Bias Rad

GCW=78,500

Bias Rad

GCW=25,000

Bias Rad

GCW=78,500

65 MPH

Tanker Trailer

Key: HP Required to Overcome –

  



  



  





Engine Brake Horsepower Required

400

300

200

100

0

55 MPH

Bias Rad

GCW=78,500

Bias Rad

GCW=25,000

Bias Rad

GCW=78,500

65 MPH

 

Aerodynamic Drag

Tire Rolling Resistance Driveline Losses

Accessory Losses   





  



  





38%

211

46%

134

282

260

23%

128

20%

41% 37%

192

42%

34%

17% 15%

32%

28%

Source: Mack Truck Engineering, Allentown, PA, Oct. 1992 Source: Goodyear Maintenance Calculations
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Factors

Affecting Truck

Fuel Economy

B. Type of Haul

The ideal type of haul for maximum

fuel economy consists of long distance

runs at steady moderate speed with a

minimum of stop-and-go driving and

with a minimum of turning. Shorter runs

involve more braking, acceleration and

turning. The engine and tires operate 

at less than optimum conditions. Fuel

economy tends to be reduced. In some

cases of stop-and-go driving, tires may

be operating “cold” part of the time

without sufficient continuous driving

time for adequate warm-up. A curve of

tire rolling resistance vs. warm-up time 

as obtained from a laboratory test is

given in Figure 4.

A 1975 study by the U.S. Department

of Transportation and the U.S. 

Environmental Protection Agencya

concluded that the type of haul (local,

short-haul, or long-haul trips) has a strong

effect on fuel economy improvement

attributable to radial tires.

The increased stop-and-go driving 

of the shorter haul reduces the fuel

economy gain due to radials. The 

results of the study are given below:

Fuel Economy Improvement Due To

Radial Tires Versus Driving Mode

Fuel Economy Driving Mode Improvement

Local 3 to 5%

Short-Haul 4 to 8%

Long-Haul 5 to 9%

aInteragency Study of Post-1980 Goals for Commercial 

Motor Vehicles; Revised Executive Summary, November 1976.

U.S. Department of Transportation and U.S. Environmental

Protection Agency.

C. Vehicle Speed

As vehicle speed is increased, 

horsepower requirements to overcome the

aerodynamic drag increase rapidly. There

is also an increase in the horsepower

required to overcome increasing tire

rolling resistance, though this occurs at

a lower rate. The sum total horsepower

requirement for a tractor-trailer vehicle

increases along a curve which has a 

continually steeper slope as speed is

increased. For example, Figure 5 shows

that the total horsepower requirement at

65 MPH is 40 percent greater than at 

55 MPH for the typical tractor and vantype trailer. As a result, fuel economy will

fundamentally decrease as operating

speed is increased from 55 to 65 MPH. 

A calculated curve of the percent difference

in MPG versus speed is shown in Figure 6.

A reduction in MPG of about eight 

percent was found for every 5 MPH

increase in vehicle speed over 55 MPH.

For 65 MPH, this would equal close to 

a mile-per-gallon loss in fuel economy.

D. Vehicle Gross

Combination Weight

As gross combination weight is

increased, tire rolling resistance increases,

and vehicle miles per gallon decreases,

assuming speed is maintained constant.

To verify this point, fuel economy

tests were conducted at the Goodyear

San Angelo Proving Grounds on

Goodyear over-the-road tractor-trailers.

Unisteel radial tires were compared to

Super Hi-Miler and Custom Cross Rib

bias ply tires on the same vehicles to

determine relative miles per gallon.a

Figure 7 shows the results of the tests

along with calculated curves passing

through the test points. The effect of

vehicle gross combination weight on miles

per gallon is shown. Note that as truck

gross weight was increased, miles per

gallon decreased with both the Unisteel

radial tire and the bias ply tire; however,

the Unisteel tire gave proportionately

greater improvement in fuel economy 

as truck gross weight was increased.

Tests were run at the San Angelo

Proving Groundsa to determine the effect

of Gross Combination Weight on vehicle

miles per gallon, comparing 11R22.5

Unisteel radial to 11-22.5 bias ply tires at

60 MPH. Figure 8 shows that at a GCW of

FIGURE 6

Vehicle Speed vs.

Percent Change in MPG & BHP

Vehicle Speed (MPH)

% Difference in MPG

30

50

40

30

20

10

0

-10

-20

-30

-40

-50

40 50 55 60 70

Vehicle Horsepower Required

Miles-Per-Gallon

FIGURE 5

Calculated 

Horsepower Requirements 

Tractor, 13.5 Ft. High Van Trailer 

vs. 

Vehicle Speed 

11R22.5 Radial Tires 

GCW=78,500 Lb.

Truck Speed–MPH

Horsepower Required

20

0

30

100

40

200

50 60 70

300

400

Accessories

Driveline Losses

Tire Roll. Resist.

Aero. Drag

Total Vehicle 

Requirements

A rule of thumb. Increase of 

10 mph = decrease of 1 mpg.

FIGURE 4

Laboratory Tests Truck Tires

Rolling Resistance

vs.

Warm-Up Time 

With Capped Air

75 PSI

Cold

95 PSI

Cold

95 PSI Hot

110 PSI Hot

11-22.5 Bias Ply

LR-F, 4760 Lb. Load

11R22.5 Radial LR-G, 5300 Lb. Load

Elapsed Time–Minutes

Uncorrected Rolling Resistance–Lb.

0 15

36

30

38

45

40

60 75 90 105

42

44

46

48

50

52

54

Source: Goodyear Maintenance Calculations

Source: Goodyear Fuel Economy Model Prediction
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Factors

Affecting Truck

Fuel Economy

78,700 lb., the measured MPG advantage

of the radial tire was 6.7 percent, while at

a GCW of 46,000 lb., the corresponding

value dropped to 1.6 percent. This

measured reduction in the miles per gallon

advantage of radial tires at the lighter

load was more severe than theory would

indicate. Calculations show that the 6.7

percent advantage should drop to about

3.5 percent at the lighter load.

aThe Effects of Goodyear Unisteel Radial Ply Tires on Fuel

Economy. Goodyear Tire & Rubber Company Booklet dated 2/77.

E. Driver

Driver operating procedures are

important factors in achieving maximum

vehicle fuel economy. The potential

benefits of lower vehicle aerodynamic

drag, lower tire rolling resistance, and

more efficient engines can be offset or

even negated by a driver running at a

higher speed.

General rules for the driver to 

follow are:b

• Keep accurate records of fuel used,

routes taken and loads carried so 

The test data above confirms that the

fuel economy advantage of radial truck

tires over bial ply tires increases with 

heavier vehicle Gross Combination

Weights.

aTire Parameter Effects of Truck Fuel Economy. R.E. Knight, 

The Goodyear Tire & Rubber Company. SAE Technical Paper

791043, November 1979.

b“17 Tricks to Save Fuel and Save $$$$”; Pamphlet DOTHS

804 547, June 1979.

you know if you are making any

improvements.

• Try progressive shifting, don’t run

against the governor on every shift

and stay 200-300 RPM below the

governor at cruise (See Figure 9).

• Stay in as high a gear as possible. 

You can’t lug today’s engines if you can

maintain speed in any gear. Keep RPM

low: below the governor but above

the minimum RPM recommended 

by the engine manufacturer.

• Eliminate unnecessary idling. Shorten

warm-up and cool-down times to 

the minimum recommended by the

engine manufacturer. Don’t leave the

engine idling while you eat lunch 

or have coffee.

• Drive defensively.

• Cut down top speed. Each MPH over

55 costs you 2.2% in fuel costs!

• Watch the fueling operation. If you

top the tank that valuable liquid could

spill or overflow later when you’re

parked in the sun.

• Carry as big a load as you can. 

Run as few empty miles as you can.

• Anticipate traffic conditions.

Accelerate and decelerate smoothly.

Tire care can also affect fuel economy.

The most important thing a driver can

do is to check inflation pressure often

with a calibrated tire gauge and make

sure that tire pressure is maintained at a

recommended high value. (See Figure 14

for effects of inflation pressure on 

fuel economy.)

FIGURE 7

Miles Per Gallon vs. Truck Gross Weight

V=55 MPH

Weight

Empty

Bias 

Tires

Radial

Tires

San Angelo Tests

12th Gear, G.R.=1.00

Texas Shuttle

10.00R20/10.00-20 Size Tires

13.5 Ft. High Van

Cummins NTC 350 Engine

 Test Points

Gross Combination Weight (Thousands of Pounds)

Miles Per Gallon

20

2.0

3.0

4.0

5.0

6.0

7.0

40 60 80 100 120 140

 ̊ 

FIGURE 9

Progressive Shifting

Governed

RPM

Miles Per Hour

0 10 20 30 40 50 60

Idle

RPM

Engine

RPM

FIGURE 8

Effect of Gross Combination Weight (GCW) 

on MPG Advantage of Radial Tires

Percent Increase in 

MPG Radial/Bias

GCW Test Data

78,700

46,000

6.7

1.6

6.7*

3.5

Calculated

*Assumed Same Value As Test Data

Source: Tricks to Save Fuel and Save $$$, DOT Pamphlet HS 804547, June, 1978

Source: Goodyear Testing Data

Source: Goodyear CFG Tests and Mathematical Calculations
S ECTION N INE

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Factors

Affecting Truck

Fuel Economy

A. Tire Rolling Resistance

The primary cause of tire rolling

resistance is the hysteresis of the tire

materials/structure, its internal friction,

which occurs as the tire flexes when the

vehicle moves. Tire rolling resistance

acts in a direction opposite the direction

of travel and is a function of both the

applied load and the tire’s inflation 

pressure (See Figure 10).

To accurately determine a tire’s rolling

resistance, a controlled laboratory test is

conducted. One method employed, is to

run the tire against an electrically driven

67'' diameter flywheel. A torque cell is

used to measure the amount of torque

required to maintain a set test speed at a

prescribed test load condition. With this

torque value, additional adjustments are

performed to arrive at the tire’s rolling

resistance. The laboratory test provides

a procedure where environmental 

influences (such as ambient temperature,

wind, and road surface texture) can be

either controlled or eliminated. Also,

strict limits are placed on allowable 

variations in test speed, slip angle, applied

load, and specified test inflation. These

controls insure test repeatability and

allow the accurate assessment of a tire’s

true rolling resistance.

Tire rolling resistance is commonly

defined in two ways:

a. Pounds resistance per 1000 pounds

of load

b. Pounds resistance per pound load

(rolling resistance coefficient)

B. Types of Tires

Radial Ply vs. Bias Ply 

The significant differences between

these two tires are the angle of body plies

and the presence of belts. Figure 11 shows

the basic structural differences. Note

that the Unisteel radial tire incorporates

a single radial ply and a multiple belt

system. The bias ply tire has six to eight

diagonally oriented plies and no belt

system (although the bias ply tire usually

has two fabric “breakers” under the 

tread with same angle as the plies). One

significant advantage of the Unisteel 

tire is the relatively low internal friction

compared to that in a tire using bias 

ply construction.

The lower internal friction of the

Unisteel tire helps minimize operating

temperatures and rolling resistance,

major causes of tire wear and excess 

fuel consumption.

Unisteel radial ply tires can provide

fuel savings of six percent and more

compared to bias ply tires in over-the-road

tractor-trailer applications.

Tubeless vs. Tube Type 

Laboratory rolling resistance tests

indicate that by changing from a 10.00R20

tube type tire to an equivalent 11R22.5

tubeless tire in all wheel positions, a

gain of about 2% in miles per gallon can

be achieved at 80,000 Ib. GCW.

Larger Diameter Tires 

Laboratory tests indicate that, under

the same load and inflation condition,

larger diameter tires produce slightly

lower rolling resistance, as in the case of

an 11R22.5 versus an 11R24.5. This can

produce an improvement in fuel economy

coupled with the reduction in engine RPMs

due to the larger overall tire diameter on

drive axles. (See Section 1-D for the

effect of engine RPMs on MPG.)

TIRE SELECTION

AND MAINTENANCE

Unisteel Radial Ply Bias Ply

FIGURE 11

Radial Ply vs. Bias Ply Construction

FIGURE 10

Load

Direction 

of Travel

Tire

Rolling

Resistance

(Tire Drag)
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Factors

Affecting Truck

Fuel Economy

Wide Base Super Single Tires

Goodyear Proving Grounds tests show

that a fully-loaded tractor-van trailer

using Goodyear Super Single Unisteel

15R22.5 tires instead of dual steel radial

11R22.5 tires on tractor drives and on

trailer, obtains an average increase of

seven to eight percent in MPG.

Commercial fleet testing using loaded

tractor-tanker trailers showed a nine 

percent gain in measured MPG through

the use of wide base single 15R22.5 steel

radial tires instead of 11R22.5 steel 

radial tires in the dual positions. A 

comparison of the super single versus

duals configuration is shown in Figure 12.

Retreaded Radial Tires 

Goodyear laboratory tests show that

the rolling resistance of newly retreaded

radial tires is, on the average, the same

as radial tires with the full original tread.

There are some differences due to type

of retread, but all newly retreaded radial

tires tested exhibited considerably lower

rolling resistance than new bias ply tires.

Radial Tires on Trailer Axles 

The type of tire used on an axle has 

a direct impact on the vehicle’s fuel

economy. Testing has shown that using

radial tires on trailer axles produces over

half of the total improvement obtained

when converting a vehicle from all bias

to all radial. Figure 13 details the total

percent gain in MPG by switching from

bias to radial tires and, of this total gain,

the percentage due to steer, drive, and

trailer tires. For maximum fuel economy

as well as for best handling, radial tires

should be used in all positions of a tractortrailer unit. Using radial tires especially

designed for trailer application will also

provide an additional improvement in

fuel economy. For example, the radial

low profile G114 offers approximately a

10 percent lower rolling resistance than

the G159 low profile.

For a vehicle already equipped with

radial tires and being switched to another

type of radial, the percent contribution

by axle to fuel economy will differ 

from that shown in Figure 13. A rule of

thumb for this case is that the front tires

contribute about 14 percent of the total,

the drive tires about 39 percent, and the

trailer tires about 47 percent. It should be

noted that the actual percent contribution

may differ from the above due to the

effects of vehicle loading, tire inflation,

and tire type.

FIGURE 13

% Difference in MPG

Bias Tires vs. Radial Tires

-ControlAll Bias

Wide Base Single

Dual Assembly

Bias

Bias

Bias

Radial

Radial

Radial

Bias

Bias

Radial

Bias

Radial

Bias

Radial

Bias

Bias

All 

Radial

RadialFronts

RadialTrailers

RadialDrives

% Gain % of

in MPG “All Radial”

vs. Control Gain in MPG

Radial Fronts 1.0% 17%

Radial Drives 1.5% 25%

Radial Trailers 3.4% 58%

FIGURE 12

Radial Wide Base Single Tire 

vs. Radial Dual Tire Assembly

% Gain in MPG vs. Control = 5.9%

Source: Goodyear CPG Test
S ECTION N INE

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Factors

Affecting Truck

Fuel Economy

C. Tire Maintenance 

Inflation Pressure 

Laboratory tests were conducted to

determine the effect of inflation pressure

on the rolling resistance of the 295/75R22.5

G159, G167, and G114 radial truck tires.

This laboratory data was used to calculate

the corresponding effect of inflation

pressure on the fuel consumption of a

typical tractor-trailer at 55 MPH on a

level highway. The effect of inflation

pressure on fuel consumption by axle

position was also studied. The results 

are shown on Figure 14. 

A dual tire load of 4250 Ibs./tire and

a steer tire load of 5390 Ibs./tire were

selected along with a specified inflation

pressure of 100 PSI for all tires. Figure 14

shows the percent loss in fuel economy

due to the lower inflation pressures.

Operating a loaded tractor-trailer

with inflation pressures of all tires as low

as 70 PSI results in a calculated reduction

in MPG of about five percent. The largest

contributor to this loss in MPG is the

reduction in inflation pressure of the

trailer tires — it alone accounts for half

the loss. Varying only the steer tire

inflation pressures results in the smallest

percent change in MPG.

It must be noted that the tractortrailer load affects the percent reduction

in MPG due to underinflation. The

lighter the GCW, the smaller the percent

loss in MPG (for the same reduction in

tire inflation).

FIGURE 14

Radial Truck Tire Inflation

vs.

Percent Change in MPG

Tire Inflation Varied:

Tire Inflation (psi)

% Difference in MPG

GCW =78,780 lbs.

V = 55 MPH 

Front Axle

Drive Axles

Trailer Axles

Front, Drive and

Trailer Axles 

60

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

-4.0

-4.5

-5.0

-5.5

-6.0

65 70 75 80 85 90 95 100 105 110 115 120

A good rule of thumb is that every

10 PSI reduction in overall tire

inflation results in about a one 

percent reduction in MPG.

Source: Goodyear Fuel Economy Model Predictions
S ECTION N INE

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Factors

Affecting Truck

Fuel Economy

Alignment 

For optimum fuel economy on a tractortrailer, and also for optimum tire wear,

tandem drive axles and tandem trailer

axles should be maintained in proper

alignment. Alignment of the vehicle’s

tandem axles should be considered as

important as the alignment of the steer

axle tires. The importance of this is not

only reflected in the loss of MPG due 

to the increase in tire rolling resistance,

but also in the increase in tire wear as 

a result of the greater amount of sidescuffing. The effects of drive axle and

trailer axle alignment is even greater 

due to the number of tires involved:

eight vs. two.

Figure 15 illustrates the results of a

Goodyear fuel economy test program run

at TRC of Ohio in 1986. These evaluations

were Type II tests conducted to SAE J1376

standards. Tests #2 and #3 with steer axle

toe-in of 1/4- inch, along with misaligned

tandem axles of 1/2-inch total (difference

in fore and aft distance between axle

center lines, from one side of the vehicle

to the other), did not result in a significant

loss in MPG versus the specification

aligned tractor-trailer. The percent

increase in tire rolling resistance due to

the slip angles (under .2°) generated by

these misalignment conditions is small.

What is of greater significance is the

loss in tire treadwear life.

Increasing the steer tire toe-in to 3/8-

inch and the tandem axle misalignment

to 1-inch in test #4 does produce a loss

in MPG which is significant.

The greatest loss in MPG was 

produced in test #5 where a “dog-tracking”

condition was simulated. The trailer 

tandem axles were misaligned by 1.5-

inch though the axles were parallel to

one another. The loss in fuel economy

was about two percent in addition to

increased tread loss.

Treadwear 

As the tread is worn down, tire

rolling resistance decreases and vehicle

fuel economy increases for both radial

and bias ply tires. Proving Grounds tests

showed about a one percent increase 

in miles per gallon for radial tires with

tread approximately 30 percent worn.a

Laboratory tests show about a 10 percent

decrease in rolling resistance for both

radial and bias ply tires with tread half

worn, and a 20 percent decrease for a

fully worn tire. (See Figure 16.) 

aTire Parameter Effects on Truck Fuel Economy by R. E.

Knight, The Goodyear Tire and Rubber Co. SAE Technical

Paper No. 791043, November 1979.

FIGURE 15

Tractor-Trailer Alignment Effects On Fuel Economy

FIGURE 16

Effect of Treadwear on Truck

Tire Rolling Resistance

Laboratory Data 

10.00R20/11R22.5 Sizes At Approx. Rated 

Dual Load And Inflation, LR-F

‘‘Tips For Truckers’’ FEA/DOT/EPA 

Document GPO 910-940 

Calspan Rep. DOT-TST-78-1

Goodyear Test

Calculation, Based On Goodyear Fuel

Economy Test

Bias Ply

Radial ply

Percent Treadwear

Percent Rolling Resistance

0

0

20

20

40

40

60

60

80

80

100

100

ALIGNMENT

Steer Tire.

Toe-In: 0'' 1/4'' 1/4'' 3/8'' 3/8''

Drive Axle.

Non-Parallel: 0'' 0'' 1/2'' 1'' 1''

Trailer Axle.

Non-Parallel: 0'' 1/2'' 1/2'' 1'' 0''

*Non-Perpendicular to Frame, 1-1/2''

% Improvement

in Fuel Economy: -0.6 -0.8 -1.7 -2.2

Test #1 Test#2 Test#3 Test #4 Test #5

Source: Goodyear Fuel Tests at TRC of Ohio, 1986
S ECTION N INE

74

Factors

Affecting Truck

Fuel Economy

ENVIRONMENTAL

CONDITIONS

A. General

Conditions external to the vehicle

can have a strong influence on the fuel

economy achieved by a given driver and

tractor-trailer/tire combination. Some of

the greater influences are exerted by: 

Winds 

Road Surface 

Ambient Temperature 

Terrain

B. Winds 

Headwinds and crosswinds reduce

truck fuel economy by increasing truck

airspeed and/or yaw angle, thus increasing

aerodynamic drag. To avoid excessive

fuel consumption in sustained strong

headwinds, a decrease in truck highway

speed is indicated.

Crosswinds also tend to diminish the

effectiveness of aerodynamic drag-reducing

devices such as cabmounted flow deflectors.

Tailwinds are generally beneficial in

increasing fuel economy because of the

reduced airspeed for a given highway

speed. However, if the driver takes

advantage of the tailwind and increases

his highway speed, the fuel economy

gains will be reduced or lost completely.

C. Road Surface 

The type of road surface can affect

tire rolling resistance. Smooth-textured

highway surfaces provide the lowest

rolling resistance, while coarse-textured

surfaces give the highest tire rolling resistance and the lowest fuel economy. 

In a test,b it was found that a coarse

chip-and-seal pavement surface gave an

increase in passenger tire rolling resistance

of 33 percent over that obtained on a

typical new concrete highway surface.

Relative rankings of the test surfaces were:

Relative 

Rolling 

Surface Resistance %

Polished Concrete 88

New Concrete 100 

Rolled Asphalt

(rounded aggregate) 101 

Rolled Asphalt (medium 

coarse aggregate) 104 

Rolled Asphalt 

(coarse aggregate) 108 

Sealed Coated Asphalt 

(very coarse) 133

Another study on passenger tiresc

investigated the effect of road roughness

(not surface texture) on rolling losses

and concluded:

1. Road roughness increases both

rolling and aerodynamic losses (the

latter due to vehicle pitching action).

2. Road roughness significantly increases

vehicle rolling losses due to energy

dissipation in the tires and suspension.

3. Tests on rough roads led to increases

in rolling losses as large as 20 percent,

in addition to introducing increases

in aerodynamic drag.

Truck fuel economy may be expected

to be influenced in a manner similar to

that of passenger cars; by the surface

condition of the roadways traveled and

by the type of materials used in the

pavement—especially in asphalt/ crushed

stone mixes. Tire treadwear as well as

vehicle fuel economy may be influenced

by the particular area of the country

being traversed, depending upon the

sharpness and hardness of the local

crushed stone used in asphaltic concrete

road pavement mixes.

bL. W. DeRAAD “THE INFLUENCE OF ROAD SURFACE

TEXTURE ON TIRE ROLLING RESISTANCE”, SAE

TECHNICAL PAPER 780257 PRESENTED AT THE

CONGRESS AND EXPOSITION, COBO HALL,

DETROIT, FEBRUARY 27 - MARCH 3, 1978.

cSteven A. Velinsky and Robert A. White, “Increased Vehicle

Energy Dissipation Due to Changes in Road Roughness with

Emphasis on Rolling Losses.” SAE Technical Paper 790653

Presented at Passenger Car Meeting, Dearborn, Michigan, 

June 11-15, 1979.
S ECTION N INE

75

Factors

Affecting Truck

Fuel Economy

D. Ambient Temperature

High ambient temperatures reduce

tire rolling resistance. High temperatures

also reduce atmospheric density, resulting

in lower aerodynamic drag. However, fuel

economy performance of non-turbocharged

diesel engines may be adversely affected

by high ambient temperatures, and this

would tend to negate some of the gains

resulting from lower tire drag and lower

aerodynamic drag.

Cold weather operation has an 

opposite effect: tire drag and aerodynamic

drag increase at the lower ambient 

temperatures. The greater thermal 

efficiency of internal combustion engines

at low ambient temperature is usually

cancelled by longer warm-up times and

longer idling times to maintain cab 

temperatures during stopover periods.

Thus, wintertime fuel economy is generally

lower than that obtained in the summer.

E. Terrain 

1. Grades 

Most proving grounds fuel economy

testing is done on level terrain, and most

simplified calculations relating various

truck and tire parameters to truck fuel

economy also assume level terrain.

The effect of traveling up a grade 

is very significant in terms of reducing

truck fuel economy. Assuming a one 

percent grade and an 80,000 pound 

tractor-trailer, there will be a rearward

force exerted by gravity of 80,000

pounds x .01 = 800 pounds.

Proving grounds tests over a measured

mile on a road with a 0.1 percent grade

consistently showed eight to ten percent

lower miles per gallon, comparing going

uphill to the west with going downhill

to the east. This difference was obtained

using a typical tractor-trailer at 55 MPH

and at a gross combination weight of

78,500 pounds.

Traveling on a downhill grade improves

fuel economy and in hilly country helps

to counteract the losses in fuel economy

sustained by traveling upgrade.

2. Altitude 

As altitude increases, air density and

atmospheric pressure decrease. At 5,000

ft. altitude, for example, air density in a

standard atmosphere is 14 percent less

than at sea level. This percent reduction

in air density also applies to reduction in

aerodynamic drag, all else being equal.

Tire rolling resistance is not affected

by altitude, per se, unless cold inflation

pressure is set at lower altitudes and not

changed as altitude of operation increases

during the course of the trip. For example,

a tire with a gauge cold inflation pressure

of 100 PSI at sea level, if taken to 5,000 ft.

altitude at the same ambient temperature,

would have a gauge cold inflation pressure

of about 103 PSI. This added inflation

would tend to reduce tire rolling resistance.

Altitude effect on engine fuel economy

performance depends on the particular

engine design and whether or not it is

supercharged or tuned for high-altitude

operation.

MPG vs. Average Daily Ambient Air Temperature

Average Daily Ambient Air Temperature

MPG

0 20 40

5.4

5.2

5

4.8

4.6

4.4

4.2

60 80 100

*

*

*

*

*

*

*

*

* *

*

** **

*

*

** ** *

* * **

** * *

*

* * *

**

*

***

*

*

*

***
S ECTION N INE

76

Factors

Affecting Truck

Fuel Economy

TIRE DESCRIPTION

AND SPECIFICATIONS

• Static Loaded Radius (SLR)—The 

distance from the road surface to 

the horizontal centerline of the wheel,

under dual load

• Minimum Dual Spacing—The minimum

dimension recommended from rim

centerline to rim centerline for optimum

performance of a dual wheel 

installation

• Loaded Section (LS)—The width of

the loaded cross-section

Tire profile or cross-sectional shape is

described by aspect ratio (AR): the ratio

of section height (SH) to section width

(SW) for a specified rim width. For a

given tire size, the aspect ratio for a

Goodyear radial truck tire is the same 

as for a bias ply truck tire.

Safety Warning

Serious Injury May Result From:

• Tire failure due to underinflation/

overloading/misapplication—follow

tire placard instructions in vehicle.

Check inflation pressure frequently

with accurate gauge.

• Explosion of tire/rim assembly due to

improper mounting—only specially

trained persons should mount tires.

When mounting tire, use safety cage

and clip-on extension air hose to inflate.

Cross-Sectional View of Typical Tire

Goodyear Unisteel 

Low Profile Radial

7. Chafer—A layer of hard rubber that

resists rim chafing.

8. Radial Ply—The radial ply, together

with the belt plies, withstands the

burst loads of the tire under operating

pressure. The ply must transmit all

load, braking, and steering forces

between the wheel and the tire tread.

9. GG Ring—Used as reference for

proper seating of bead area on rim.

10. Bead Core—Made of a continuous

high-tensile wire wound to form a

high-strength unit. The bead core is

the major structural element in the

plane of tire rotation and maintains

the required tire diameter on the rim.

Terms Used To Describe Tire/

Rim Combination

• Outside Diameter (OD)—The

unloaded diameter of the tire/

rim combination

• Section Width (SW)—The maximum

width of the tire section, excluding

any lettering or decoration

• Section Height (SH)—The distance

from the rim to the maximum height

of the tire at the centerline

1. Tread—This rubber provides the

interface between the tire structure

and the road. Primary purpose is to

provide traction and wear.

2. Belts—Steel cord belt plies provide

strength to the tire, stabilize the

tread, and protect the air chamber

from punctures.

3. Stabilizer Ply—A ply laid over 

the radial ply turnup outside of the

bead and under the rubber chafer

that reinforces and stabilizes the

bead-to-sidewall transition zone.

4. Sidewall—The sidewall rubber must

withstand flexure and weathering

while providing protection for 

the ply.

5. Liner—Layers of rubber in tubeless

tires especially compounded for

resistance to air diffusion. The liner

in the tubeless tire replaces the

innertube of the tube-type tire.

6. Apexes—Rubber pieces with 

selected characteristics are used to

fill in the bead and lower sidewall

area and provide a smooth transition

from the stiff bead area to the 

flexible sidewall.

1

3

5

6

4

8

9

2

7

10

Section

Width (SW)

Static Loaded

Radius (SLR)

Flange

Height

Rim 

Width

Outside

Diameter 

(OD)

Section 

Height (SH)

Minimum 

Dual 

Spacing

Loaded Section

(LS)
S ECTION N INE

77

Factors

Affecting Truck

Fuel Economy

SUMMARY

The average fuel costs of a given

trucking fleet are related to two factors:

• Average fleet miles per gallon

• Average fuel cost per gallon

While it seems little can be done at

the present time to reduce fuel cost per

gallon, there are steps that can be taken

to increase average fleet miles per gallon. 

The miles per gallon achieved by a

given truck depends on many factors,

the major ones being:

• Vehicle, Engine and Accessory Design

and Maintenance

• Vehicle Operation

• Tire Selection and Maintenance

• Environmental Conditions

Major fuel-saving steps to apply to

trucking operations are: 

1. Use fuel-efficient high torque rise,

lower RPM engines.

2. Use engine accessories with reduced

horsepower requirements, such as

clutch fans, synthetic lubricants, etc.

3. Use aerodynamic drag reduction

devices such as flow deflectors and

rounded trailer fronts and corners 

on tractors pulling van-type trailers.

Cover open-topped trailers with a

tightly-stretched tarpaulin.

4. Use radial tires in all wheel positions,

trailer as well as tractor.

5. For best fuel economy, do not allow

radial tires to operate below 95 PSI

cold inflation pressure.

6. Do not exceed the tire’s rated speed; 

operate truck fully loaded as much 

of the time as possible to increase

ton-miles per gallon.
S ECTION N INE

78

Factors

Affecting Truck

Fuel Economy

APPENDIX

Fuel Economy Test Procedures

There are three fuel economy test

procedures which have been developed

by the Society of Automotive Engineers

(SAE) and which are currently being

used by vehicle manufacturers, tire 

manufacturers, and by some fleet owners.

These offer a standardized method to

evaluate either a complete vehicle or a

component. Consideration has been given

to the effects of environmental conditions

(such as those described in Section 4),

and their effect on fuel economy results.

This is accomplished by requiring the

use of a control vehicle which is run

simultaneously with the test vehicle.

Environmental conditions should affect

both vehicles in a similar manner so that

for a set of tests, the ratio of either the

fuel used or the MPG of the test and

control vehicles should be relatively 

constant even though the actual values

of either the fuel used or the MPG may

vary from test to test. 

A brief description of each procedure

is listed along with some of their important

requirements.

A. SAE Type I

The SAE Type I procedure is best used

to evaluate a component which can be

easily switched from one vehicle to another.

The procedure requires two vehicles

of the same specification; these are run

simultaneously and are identified as 

vehicles “A” and “B.”

The minimum mileage required for

one complete test cycle is 200 miles.

This is composed of a 100 mile round

trip with the test component on vehicle

“B” and then another 100 mile round trip

with the test component on vehicle “A.”

Since a round trip must start and finish

at the same location, the minimum

length of the outbound and inbound 

test leg is 50 miles.

On the outbound test leg vehicle “A”

leads vehicle “B” (approximately 200 -

250 yard separation). At a point halfway

through this test leg (approx. 25 miles)

vehicle “A” slows down to allow vehicle

“B” to take the lead. At the completion of

the outbound leg, fuel tanks are weighed

or fuel meter readings are recorded. On

the inbound test leg, vehicle “B” leads “A”

(same separation distance as outbound

leg). Also at a point halfway through the 

test leg “B” slows down to allow “A” to

take the lead. Upon completion, fuel is

weighed or meters recorded. The test

component is then switched between

vehicles and another round trip is made.

The amount of fuel used by vehicles

“A” and “B” when they are operating 

with the test component is compared to

that used by both vehicles without the

test component.

Test speed — as required

Vehicle loads — within five percent 

of each other

Vehicle 

warm-up — representative of fleet

operation or not less

than 45 minutes at

test speed

B. SAE Type II 

The SAE Type II procedure is best

used to evaluate a component which

requires a substantial amount of time 

for removal and replacement.

This procedure also requires two

vehicles, though they do not have to be

of the same specification. The vehicles

are identified as “C” and “T.” Vehicle “C”

is the control vehicle and as such is not

modified during the course of the test;

vehicle “T” is the test vehicle which is

used to evaluate the test component.

The minimum mileage for a complete

test is 240 miles. This is composed of

three valid test runs of 40 miles (minimum)

each with vehicle “T” running a baseline

component (control component) and

then three valid test runs of 40 miles

(minimum) each with vehicle “T” running

the test component. Vehicle “T” starts 

off first; after approximately 5 minutes

vehicle “C” begins its run. The test run

starts and finishes at the same location.

For each test run the amount of fuel

used by vehicle “T” is compared to that

used by vehicle “C” in the form of a T/C

ratio—the quantity of fuel used by vehicle

“T” divided by the quantity of fuel used

by vehicle “C.” To be considered valid

test runs, three T/C ratios within a two

percent band must be obtained. This may

require one or more additional test runs.

Test speed — as required 

Vehicle loads — not required to 

be the same

Vehicle 

warm-up — minimum of one

hour at test speed 

Test run time — elapsed time of the

test runs must be

within .5%
S ECTION N INE

79

Factors

Affecting Truck

Fuel Economy

C. SAE Engineering Type

The SAE Engineering Type test provides

standardized procedures to evaluate fuel

economy for different modes of operation,

such as Long Haul Cycle, Short Haul

Cycle, Local Cycle, and Transit Cycle.

This procedure is more controlled than

either the Type I or II tests both in terms

of test site conditions and test procedures.

The effect of this is reflected in greater

repeatability. This procedure is best run

on a test track.

The procedure requires two vehicles

preferably of the same specification. 

The vehicles are identified as “C” and “T.”

Vehicle “C” is the control vehicle and is

not modified during the course of the

test. Vehicle “T” is the test vehicle which

is used to evaluate the test component.

Long Haul Cycle: 

The minimum mileage for a complete

test is 180 miles. This is composed of

three valid 30 mile test runs with vehicle

“T” running the baseline component

(control component) and three valid test

runs with “T” running the test component.

The start time of the vehicles should 

be staggered such that they don’t 

aerodynamically interfere with each

other. Halfway through each test run 

(15 miles) the vehicles are to come to a

complete stop, idle for one minute and

then accelerate back to the test speed. 

A test run starts and finishes at the same

location. If this procedure is not run on 

a track it can be handled by running 

15 miles outbound and 15 miles inbound.

For each test run a T/C ratio is

obtained. This is the MPG of vehicle

“T” divided by the MPG of vehicle “C.”

A test is considered valid if for the three

runs (or more) the spread of T/C ratios

doesn’t exceed three percent of the

mean value.

Test speed — 55 MPH 

Vehicle loads — as required

Ambient 

temperature — 60 to 80°

Wind velocity — average wind speed

not to exceed 

15 MPH

Vehicle 

warm-up — minimum of 1 hour

at 55 MPH