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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD

ANN ARBOR, MICHIGAN 48105-2498

 

MEMORANDUM

SUBJECT: Draft Baseline GEM Inputs for Tractors, Trailers, and Vocational Vehicles

FROM: Charles Moulis, Senior Environmental Engineer

Assessment and Standards Division

TO: Docket Number EPA-HQ-OAR-2014-0827

DATE: March 24, 2016

On March 2, 2016, the agencies issued a Notice of Data Availability (NODA) and placed into the 

public docket a revised version of the Greenhouse Gas Emission Model (GEM) that reflects 

input received during the public comment period and continued analytical work. Some 

stakeholders have requested additional information to assist in the determination of the baseline 

vehicle performance. This memorandum summarizes the draft input values for this revised 

version of GEM that reflect the agencies’ current projection of average values for the model year 

2018 baseline heavy-duty fleet. In other words, running the revised version of GEM using these 

inputs will show the average levels of performance the agencies would expect to see from 

tractors and vocational vehicles that are fully compliant with the 2018 Phase 1 standards. The 

input values presented here reflect our projections based on currently available information. 

However, we continue to look for new information that would improve our projections. Thus, it 

is possible that the baseline values that will be used for the final rules will differ from the values 

presented here.

Baseline inputs are presented separately here for tractors, trailers and vocational vehicles. The 

simpler input values such as tire rolling resistance and axle ratios are shown in the tables below. 

Note that the tables may contain information for more than one regulatory subcategory. More 

complex inputs such as axle efficiency maps are attached as data files or were included with the 

GEM release.

The tractor inputs are shown in Table 1. The tractor table does not include the aerodynamic 

inputs for high roof tractors because the agencies are modifying the aerodynamic test procedures 

for high roof tractors in response to the comments received from the Phase 2 Notice of Proposed 

Rulemaking and have not yet completed the analysis of these changes on the inputs. The test 

procedure changes are being made to better isolate aerodynamic drag from other road load 

components and to reduce variability in test results. Although we cannot provide numerical 

values at this time, we can note that isolating the aerodynamic drag would make the new CdA

values lower than the proposed values (see 80 FR 40220). Thus, we expect the baseline 

aerodynamic value for high roof sleeper cabs to be less than the proposed 6.22 m2value. The 

2017 MY engine configuration files (including the fuel maps) and a 10 speed manual 

OFFICE OF

AIR AND RADIATION
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transmission configuration file were included in the release of GEM included in the NODA. 

The agencies are posting an updated baseline axle configuration file for tractors and heavy 

heavy-duty vocational vehicles with this memo. 

Table 1 Tractor Inputs

Class 7 Class 8

Day Cab Day Cab Sleeper Cab

Low 

Roof

Mid 

Roof

High 

Roof Low Roof Mid Roof

High 

Roof Low Roof Mid Roof High Roof

Engine

2017 MY 

11L 

Engine 

350 HP

2017 MY 

11L 

Engine 

350 HP

2017 MY 

11L 

Engine 

350 HP

2017 MY 

15L 

Engine 

455 HP

2017 MY 

15L 

Engine 

455 HP

2017 MY 

15L 

Engine 

455 HP

2017 MY 

15L 

Engine 

455 HP

2017 MY 

15L 

Engine 455 

HP

2017 

MY 

15L 

Engine 

455 HP

Aerodynamics (CdA in m2)

5.00 6.40 30% Bin 

II

60% Bin 

III

10% Bin 

IV

5.00 6.40 30% Bin 

II

60% Bin 

III

10% Bin 

IV

4.95 6.35 Bin III

Steer Tires (CRR in kg/metric ton)

6.99 6.99 6.87 6.99 6.99 6.87 6.87 6.87 6.54

Drive Tires (CRR in kg/metric ton)

7.38 7.38 7.26 7.38 7.38 7.26 7.26 7.26 6.92

Extended Idle Reduction – Adjustable AESS with no Idle Red Tech Adoption Rate @ 1% Effectiveness

N/A N/A N/A N/A N/A N/A 87% 87% 87%

Extended Idle Reduction – Adjustable AESS with Diesel APU Adoption Rate @ 3% Effectiveness

N/A N/A N/A N/A N/A N/A 9% 9% 9%

Transmission = 10 Speed Manual Transmission

Gear Ratios = 12.8, 9.25, 6.76, 4.90, 3.58, 2.61, 1.89, 1.38, 1.00, 0.73

Drive Axle Configuration = 4x2

Use GEM Default

Drive Axle Configuration = 6x4

Use 6x4 Axle Configuration File Provided with this Memo

Tire Revs/Mile = 512

Drive Axle Ratio = 3.42
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The current default trailer parameters are shown in Table 2. These values are EPA-specified 

values that remain constant in the GEM simulations. The default CdA values are shown as a 

range. In response to several commenters, the agencies are considering the use of wind-averaged 

drag results, which will impact the absolute CdA value used in the GEM. Similar to the 

proposal, each trailer subcategory has a distinct default CdA value to reflect the aerodynamic 

performance of the specific tractor-trailer combination. 

Table 3 shows the GEM inputs used to represent the baseline trailers. We proposed full-, partialand non-aero subcategories for our van trailers and we have created separate baselines for each 

of these subcategories. The aerodynamic inputs for the full-aero long box trailers are based on a 

slightly higher adoption of aero devices that perform similarly to skirts, while short box trailers 

and those with work-performing devices that would qualify as partial- or non-aero vans are 

assumed to have no adoption of aerodynamic devices. The baseline tire rolling resistance 

includes higher adoption of SmartWay-verified tires based on information we received in 

comments. The baseline weight reduction and tire pressure system adoption are unchanged from 

the proposal.

Note that non-box trailers are not shown in Table 2 or Table 3, because we proposed to apply 

non-GEM standards to those trailers. 

Table 2 Default Trailer Parameters in GEM

Regulatory Subcategory Long Box Van Short Box Van

Dry Refrigerated Dry Refrigerated

Representative Trailer Length 53-foot 28-foot

Standard Tractor Class 8 high-roof sleeper cab Class 8 high-roof day cab

Steady-State Engine Map 15 L, 455 HP

Total vehicle weight (kg) 31978 33778 15191 16991

Default CdA Values - TBD (m2) 5.8 - 6.2

Number of Axles 5 4

Payload (tons) 19 10

CARB HHDDT Drive Cycle Weighting 0.05 0.19

GEM 55 mph Drive Cycle Weighting 0.09 0.17

GEM 65 mph Drive Cycle Weighting 0.86 0.64

Table 3 Baseline Trailer Inputs

Full-Aero, Long Box Full-Aero, Short Box All Partial- and 

Dry Refrigerated Dry Refrigerated Non-Aero Vans

Delta CdA (m2) 0.2 0.2 0 0 0

CRR (kg/ton) 5.2 5.2 5.2 5.2 6.0

Weight Reduction (lb) 0 0 0 0 0

Tire Pressure System Credit (%) 0.5 0.5 0.3 0.3 0
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Vocational vehicle draft baseline vehicle inputs are shown in Table 4 through Table 11. The 

values not shown in the tables are GEM-default values unchanged from the proposal, such as 

payload, drag area, and vehicle weight. The agencies are currently considering public comments 

received regarding the default drag area for light heavy-duty vocational vehicles, and although 

we expect the final default drag area to be less than the proposed value of 5.4 m2, we have not 

yet completed the analysis necessary to establish a new CdA value for these vehicles.

Table 4 Vocational LH Diesel Inputs

GEM Parameter Regional Multi-Purpose Urban

CI Engine 2018 MY 7L, 200 hp Engine

Transmission Type 6-speed AT 6-speed AT 5-speed AT

Transmission Gears 3.102, 1.8107, 1.4063, 1.0, 

0.7117, 0.61

3.102, 1.8107, 

1.4063, 1.0, 

0.7117

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3 3

Axle efficiency improvement Use GEM Default

Drive Axle Gear Ratio 4.1 4.56 4.56

Axle Configuration 4x2 4x2 4x2

Idle Reduction No

Tire Revs/mile 622 622 622

Steer Tires (CRR kg/metric ton) 7.7 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7 7.7

Aerodynamic Improvement 0 0 0

Electrified Accessories 0 0 0

Tire Pressure System 0 0 0

PTO Improvement 0 0 0

Weight Reduction (lb) 0 0 0
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Table 5 Vocational LH Gasoline Inputs

GEM Parameter Regional Multi-Purpose Urban

SI Engine 2018 MY 6.8L, 300 hp engine

Transmission Type 6-speed AT 6-speed AT 5-speed AT

Transmission Gears 3.102, 1.8107, 1.4063, 1.0, 

0.7117, 0.61

3.102, 1.8107, 

1.4063, 1.0, 

0.7117

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3 3

Axle efficiency improvement Use GEM Default

Drive Axle Gear Ratio 4.33 4.88 4.88

Axle Configuration 4x2 4x2 4x2

Idle Reduction No

Tire Revs/mile 557 557 557

Steer Tires (CRR kg/metric ton) 7.7 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7 7.7

Aerodynamic Improvement 0 0 0

Electrified Accessories 0 0 0

Tire Pressure System 0 0 0

PTO Improvement 0 0 0

Weight Reduction (lb) 0 0 0
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Table 6 Vocational MH Diesel Inputs

GEM Parameter Regional Multi-Purpose Urban

CI Engine 2018 MY 7L, 270 hp Engine

Transmission Type 6-speed AT 6-speed AT 5-speed AT

Transmission Gears 3.102, 1.8107, 1.4063, 1.0, 

0.7117, 0.61

3.102, 1.8107, 

1.4063, 1.0, 

0.7117

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3 3

Axle efficiency improvement Use GEM Default

Drive Axle Gear Ratio 4.33 4.88 4.88

Axle Configuration 4x2 4x2 4x2

Idle Reduction No

Tire Revs/mile 517 517 557

Steer Tires (CRR kg/metric ton) 7.7 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7 7.7

Aerodynamic Improvement 0 0 0

Electrified Accessories 0 0 0

Tire Pressure System 0 0 0

PTO Improvement 0 0 0

Weight Reduction (lb) 0 0 0
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Table 7 Vocational MH Gasoline Inputs

GEM Parameter Regional Multi-Purpose Urban

SI Engine 2018 MY 6.8L, 300 hp engine

Transmission Type 6-speed AT 6-speed AT 5-speed AT

Transmission Gears 3.102, 1.8107, 1.4063, 1.0, 

0.7117, 0.61

3.102, 1.8107, 

1.4063, 1.0, 

0.7117

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3 3

Axle efficiency improvement Use GEM Default

Drive Axle Gear Ratio 4.33 4.88 4.88

Axle Configuration 4x2 4x2 4x2

Idle Reduction No

Tire Revs/mile 517 517 517

Steer Tires (CRR kg/metric ton) 7.7 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7 7.7

Aerodynamic Improvement 0 0 0

Electrified Accessories 0 0 0

Tire Pressure System 0 0 0

PTO Improvement 0 0 0

Weight Reduction (lb) 0 0 0
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Table 8 Vocational HH Diesel Inputs

GEM Parameter Regional Multi-Purpose Urban

CI Engine 2018 MY 15L 

455hp Engine

2018 MY 11L, 345 hp Engine

Transmission Type 10-speed MT 5-speed AT

Transmission Gears 12.8, 9.25, 

6.76, 4.9, 

3.58, 2.61, 

1.89, 1.38, 

1.0, 0.73

4.6957, 2.213, 1.5291, 1.0, 

0.7643

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3 3

Axle efficiency improvement Use 6x4 Axle Configuration File Provided with 

this Memo

Drive Axle Gear Ratio 3.76 4.33 5.29

Axle Configuration 6x4 6x4 6x4

Idle Reduction No

Tire Revs/mile 496 496 496

Steer Tires (CRR kg/metric ton) 7.7 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7 7.7

Aerodynamic Improvement 0 0 0

Electrified Accessories 0 0 0

Tire Pressure System 0 0 0

PTO Improvement 0 0 0

Weight Reduction (lb) 0 0 0
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Table 9 Vocational HH Gasoline Inputs

GEM Parameter Regional Multi-Purpose Urban

SI Engine 2018 MY 6.8L, 300 hp engine

Transmission Type 6-speed AT 6-speed AT 5-speed AT

Transmission Gears 4.6957, 2.213, 1.5291, 1.0, 

0.7643, 0.6716

4.6957, 2.213, 

1.5291, 1.0, 

0.7643

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3 3

Axle efficiency improvement Use 6x4 Axle Configuration File Provided with 

this Memo

Drive Axle Gear Ratio 4.1 4.33 4.88

Axle Configuration 6x4 6x4 6x4

Idle Reduction No

Tire Revs/mile 496 496 496

Steer Tires (CRR kg/metric ton) 7.7 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7 7.7

Aerodynamic Improvement 0 0 0

Electrified Accessories 0 0 0

Tire Pressure System 0 0 0

PTO Improvement 0 0 0

Weight Reduction (lb) 0 0 0

Stakeholders interested in simulating custom vocational vehicles should enter GEM inputs

according to the configurations described below in Table 10 and Table 11. The release of GEM 

included in the NODA will give results for the agencies’ current estimates of these baseline 

vehicles if the simulations are run using the following regulatory subcategory classifications: 

Motor homes: MHD_R

School bus: MHD_U

Coach Bus: HHD_R

Transit, Refuse, Mixer, Emergency: HHD_U
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Table 10 Custom Vocational MHD Inputs

GEM Parameter Motor Homes 

(Regional)

School Bus 

(Urban)

CI Engine 2018 MY 7L, 270 hp Engine

Transmission Type 6-speed AT 5-speed AT

Transmission Gears 3.102, 1.8107,

1.4063, 1.0, 

0.7117, 0.61

3.102, 1.8107, 

1.4063, 1.0, 0.7117

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3

Axle efficiency improvement Use GEM Default

Drive Axle Gear Ratio 4.33 4.88

Axle Configuration 4x2 4x2

Idle Reduction No No

Tire Revs/mile 517 517

Steer Tires (CRR kg/metric ton) 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7

Aerodynamic Improvement 0 0

Electrified Accessories 0 0

Tire Pressure System 0 0

PTO Improvement 0 0

Weight Reduction (lb) 0 0
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Table 11 Custom Vocational HHD Inputs

GEM Parameter Coach Bus 

(Regional)

Transit, Refuse, Mixer, 

Emergency (Urban)

CI Engine 2018 MY 11L, 345 hp Engine

Transmission Type 6-speed AT 5-speed AT

Transmission Gears 4.6957, 2.213, 

1.5291, 1,

0.7643, .6716

4.6957, 2.213, 1.5291, 

1, 0.7643

Transmission efficiency 

improvement

Use GEM Default

Torque converter lockup gear 3 3

Axle efficiency improvement Use 6x4 Axle Configuration File Provided 

with this Memo

Drive Axle Gear Ratio 4.33 5.29

Axle Configuration 6x4 6x4

Idle Reduction No No

Tire Revs/mile 496 496

Steer Tires (CRR kg/metric ton) 7.7 7.7

Drive Tires (CRR kg/metric ton) 7.7 7.7

Aerodynamic Improvement 0 0

Electrified Accessories 0 0

Tire Pressure System 0 0

PTO Improvement 0 0

Weight Reduction (lb) 0 0