Greenhouse Gas Emissions Model 

(GEM) User Manual 

Vehicle Simulation Tool for Compliance 

with the Proposed Greenhouse Gas 

Emissions Standards and Fuel Efficiency 

Standards for Medium and Heavy-Duty 

Engines andVehicles: Phase 2 
 

Greenhouse Gas Emissions Model
 

(GEM) User Manual
 

Vehicle Simulation Tool for Compliance 


with the Proposed Greenhouse Gas 


Emissions Standards and Fuel Efficiency 


Standards for Medium and Heavy-Duty 


Engines andVehicles: Phase 2
 

Assessment and Standards Division 

Office of Transportation and Air Quality 

U.S. Environmental Protection Agency 

EPA-420-B-15-076 

June 2015 
 

 

 

 

Table of Contents 

Table of Contents................................................................................................................ 3
 

Introduction......................................................................................................................... 4
 

Installation........................................................................................................................... 4
 

Computer Requirements ................................................................................................. 4
 

Installation Instructions................................................................................................... 4
 

Contents of Installation Package..................................................................................... 9
 

Sample Input Files ........................................................................................................ 10
 

GEM Executable........................................................................................................... 11
 

Model Description ............................................................................................................ 11
 

Vehicle Parameters for Each Regulatory Subcategory................................................. 13
 

Drive Cycles and Transient Correction Factor ......................................................... 13
 

Tractor Vehicle Parameters....................................................................................... 14
 

Vocational Vehicle Parameters................................................................................. 16
 

Trailer Vehicle Parameters........................................................................................ 19
 

GEM Input File Structure ................................................................................................. 21
 

Note to Users Creating or Editing .csv Files Using Microsoft Excel ........................... 22
 

Tractor Input Files......................................................................................................... 22
 

Vocational Input Files................................................................................................... 26
 

Trailer Input Files ......................................................................................................... 30
 

Supplemental Input Files .............................................................................................. 32
 

Engine Input File for Tractor and Vocational Vehicles............................................ 32
 

Transmission Input File for Tractor and Vocational Vehicles.................................. 34
 

Optional Powertrain Input File for Tractor and Vocational Vehicles....................... 34
 

GEM Output File Structure............................................................................................... 36
 

Running GEM................................................................................................................... 36
 

Preparing for GEM Runs .............................................................................................. 37
 

Special Instructions for Heavy-Haul Tractor Manufacturers.................................... 37
 

Special Instructions for Vocational Emergency Vehicle Manufacturers.................. 37
 

Accessing Output Files ................................................................................................. 37
 

Running GEM Using the Start Menu and Desktop Icon .............................................. 37
 

Running GEM Using the Command Prompt................................................................ 42
 

Running GEM from Microsoft Excel ........................................................................... 46
 

Final Notes........................................................................................................................ 48
 

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Introduction 

The Greenhouse gas Emissions Model (GEM) was first created by EPA as part of the 

“Heavy-Duty Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Mediumand Heavy-Duty Engines and Vehicles: Phase 1” rulemaking finalized in 2011. The model was 

developed to serve as a means for determining compliance with EPA’s GHG emissions and 

NHTSA’s fuel consumption vehicle standards for Class 7 and 8 combination tractors and Class 

2b-8 vocational vehicles. Since then, significant improvements have been made to the model for 

the Phase 2 proposed rulemaking. 

This user guide contains the model documentation, including a summary of the upgrades 

from GEM version 2.0.1 (i.e., “Phase 1 GEM”), installation instructions, and instructions for 

running the model, including a description of the necessary input and resulting output files. A 

detailed description of the model architecture and updates can be found in Chapter 4 of the draft 

RIA. 

Installation 

Computer Requirements 

EPA developed Phase 2 GEM to be a forward-looking Matlab/Simulink-based model for 

heavy-duty (Class 2b-8) vehicle compliance for the Phase 2 proposed rulemaking. The model is 

a free, desktop computer application provided as an executable to be operated on a single 

computer. Since it is provided as an executable, the user does not have to have access to the 

Matlab/Simulink software packages. However, the following minimum computer specifications 

are required for the model to run: 

– Operating System: Windows 7 or newer 

– CPU: 2 GHz processor 

– Memory: 4 GB of RAM 

Installation Instructions 

The downloadable installation file is available on EPA’s website (see Figure 1) at: 

http://www.epa.gov/otaq/climate/gem.htm. 

A link to the most recent GEM version will be located at the top of the page. The GEM 

executable is bundled with the user guide and input file templates into a 600 MB file that can be 

downloaded in a single step. The user guide is also available on the website for convenience. 

To request a CD of this software instead of downloading it, or to request assistance if you 

have trouble with accessibility of this software, please request through an email addressed to 

OTAQPUBLICWEB@epa.gov. 

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Figure 1: EPA Website to Obtain GEM Installation Package 

Currently, GEM is only available to computers using 64-bit Windows operating systems 

(Windows 7 and newer). To check your computer’s operating system, right click on “Computer” 

from a desktop icon or the Windows Start Menu to pull up the System Properties window. If 

your computer has a 64-bit operating system, it will be noted on the properties window, as is 

seen in Figure 2. 

Figure 2: System Properties for a 64-bit Operating System (Newer than Windows XP) 

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To install GEM, download the installation file from EPA’s website. Once downloaded, 

double-click the “GEM_P2v1.0_Setup_x64” executable to start the setup wizard that will walk 

you through the steps of installation. 

Figure 3: Welcome Window for the Setup Wizard for Phase 2 GEM 

Users have the option of choosing a separate location for their Phase 2 GEM installation, 

but we recommend the default folders and shortcuts, as seen in Figure 4 and Figure 5. The 

instructions throughout this user guide assume the user installed Phase 2 GEM in the default 

locations with its default folder names. 

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Figure 4: Destination for Phase 2 GEM Download; the Agencies Recommend the Default Location 

Figure 5: Start Menu Folder for Phase 2 GEM Download; the Agencies Recommend the Default Folder 

Name 

The next screen gives users an option to create a shortcut to Phase 2 GEM on their 

desktop. Users that choose to install a desktop icon will see a “Phase 2 GEM” shortcut, similar 

to the one shown in Figure 6, on their desktop. Users can uncheck the box next to the “Create a 

desktop icon” if they do not wish to have a desktop shortcut. 

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Figure 6: Desktop Icon Shortcut to Phase 2 GEM 

Phase 2 GEM requires the use of Matlab Runtime Compiler and Microsoft Visual C++ 

2005 or 2008 Redistributable (x64). The setup wizard will install Matlab Runtime Compiler 

R2014a (version 8.3) if the box, shown in Figure 7, is checked. For computers that already have 

the runtime compiler (R2014a or newer) installed, users can uncheck this box to avoid 

reinstallation. 

Figure 7: Additional Installation Options for Phase 2 GEM; Matlab Runtime Compiler R2014a or Newer is 

Required for GEM Use 

The following windows allow users to review the installation settings and install Phase 2 

GEM. The installer will warn users if their computers do not have Microsoft Visual C++ 2005 

or 2008 Redistributable (x64) installed. A pop-up window will initiate installation of this 

software and will remain displayed until installation is complete. 

A final screen (Figure 8) will show up when Phase 2 GEM has completed installing. The 

installation process for Matlab Runtime Compiler (if selected) will initiate shortly after this 

completion screen appears. The Matlab setup wizard for the runtime compiler is not described in 

this user guide. 

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Figure 8: Installation Complete Window for Phase 2 GEM 

Contents of Installation Package 

Once installed, several files are stored in the installation location selected (i.e., 

C:\Program Files\US EPA\Phase 2 GEM\ by default). Figure 9 shows the contents of the default 

installation folder. Users can access the GEM User Guide, the GEM executables, and sample 

files from this directory. They can also uninstall GEM and remove all of its contents by clicking 

on the “unins000.exe” executable. 

Figure 9: Contents of Installation Folder for Phase 2 GEM 

GEM is also available from the Start Menu, under the folder named “EPA Phase 2 

GEM”, as seen in Figure 10. Within the Start Menu folder, users have access to the executable, 

as well as a copy of this User Guide, a link to the EPA website, and sample input files that can be 

used as templates for running GEM. 

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Figure 10: Start Menu Location of GEM 

Sample Input Files 

Sample input files are stored with the Phase 2 GEM executable. Within the “Sample 

Input Files – RELOCATE BEFORE USE” folder, the agencies provide sample files for each of 

the three vehicle regulatory subcategories that use GEM: Tractor, Trailer, and Vocational 

vehicles. The following documents will be available in three folders: 

– Tractor sample files folder 

o Sample engine input file: EPA_Sample_350hp.csv 

o Sample engine input file: EPA_Sample_455hp.csv 

o Sample transmission input file: EPA_Sample_AMT.csv 

o Sample transmission input file: EPA_Sample_MT.csv 

o Sample vehicle input file: GEM_TRACTOR.csv 

o Sample vehicle Excel macro file: GEM_TRACTOR.xlsm 

– Trailer sample files folder 

o Sample vehicle input file: GEM_TRAILER.csv 

o Sample vehicle Excel macro file: GEM_TRAILER.xlsm 

– Vocational sample files folder 

o Sample engine input file: EPA_Sample_200hp.csv 

o Sample engine input file: EPA_Sample_270hp.csv 

o Sample engine input file: EPA_Sample_345hp.csv 

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o Sample engine input file: EPA_Sample_455hp.csv 

o Sample transmission input file: EPA_Sample_AT_5_HHD.csv 

o Sample transmission input file: EPA_Sample_AT_5_MLHD.csv 

o Sample transmission input file: EPA_Sample_MT_10_HHD.csv 

o Sample vehicle input file: GEM_VOCATIONAL.csv 

o Sample vehicle Excel macro file: GEM_VOCATIONAL.xlsm 

A description of each of these input files and instructions for running the model are 

provided in later sections. 

If users select the default installation folder, C:\Program Files\US EPA\Phase 2 GEM\, it 

is recommended that they copy the folder “Sample Input Files – RELOCATE BEFORE USE” 

from this default folder to a local folder. By doing so, users can easily run GEM and write output 

results into the selected local folder and avoid potential administrative rights issues, since not all 

organizations allow users to write files to C:\Program Files folders. Users can copy the sample 

files folder to any convenient local folder and can rename it as needed once it is copied to its new 

destination. 

GEM Executable 

The agencies propose to require that vehicle manufacturers use the Phase 2 GEM 

executable, which does not require the use of Matlab or Simulink software, for demonstrating 

compliance with the proposed CO2 and fuel consumption standards. Phase 2 GEM does not offer 

a graphical user interface (GUI) for users to provide their vehicle parameters. Instead, inputs are 

provided in a .csv file. However, the agencies do provide a simple Microsoft Excel file with a 

macro built in, allowing individual users or manufacturers to generate the necessary input file 

and run GEM with one button-click. Results are available in a generated report that can be 

viewed using either a text editor or spreadsheet. The following sections will describe the model, 

its input files, and its output files in more detail. 

Model Description 

Phase 1 GEM was updated in order to meet proposed Phase 2 rulemaking requirements. 

Phase 2 GEM improves the fidelity of the Phase 1 model to better match the function of the 

simulated vehicles and accurately reflect changes in technology for compliance purposes. Many 

of the modifications were the result of numerous constructive comments from both public 

comments and GEM peer reviews1

. The following sections describe the model with an emphasis 

on the additional vehicle parameters available in the Phase 2 upgrade of the model. Users are 

directed to Chapter 4 of the draft RIA for more detailed information regarding the model 

architecture and validation. 

GEM Architecture and Summary of Upgrades 

The GEM architecture is comprised of four systems: Ambient, Driver, Powertrain, and 

Vehicle as seen in Figure 11. The Powertrain and Vehicle systems consist of one or more 

subcomponent models and a description of the subcomponent models is available in Chapter 4 of 

the draft RIA. 

11 
 

Figure 11: GEM model structure 

The agencies are proposing additional regulatory subcategories to better represent the 

heavy-duty vehicles and these subcategories are reflected in the model with additional vehicle 

models. Phase 2 GEM also incorporates the proposed improvements to the duty cycles, 

including the addition of an idle cycle for vocational vehicles, and modified cruise cycles that 

account for changes in road grade. Specifically, the agencies are proposing to implement the 

following key technical features into Phase 2 GEM: 

•	 An upgraded engine controller, which includes engine fuel cut-off during braking and 

deceleration 

•	 An upgraded transmission model, which includes an upgraded manual transmission, 

along with newly developed automatic and automated manual transmissions 

•	 An upgraded driver model with a distance-compensated driver that will drive the 

certification drive trace over a prescribed distance regardless of increased drive time due 

to vehicle under-performance, for example 

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With these upgrades, the model is capable of recognizing most technologies that could be 

evaluated in both engine and chassis dynamometers and is better able to reflect changes in 

technologies for compliance purposes. See Chapter 4 of the draft RIA for more information 

about these upgrades. 

Vehicle Parameters for Each Regulatory Subcategory 

GEM is a flexible simulation platform that can model a wide variety of vehicles with 

conventional powertrains from Class 2b to Class 8. The key to this flexibility is the component 

description files that can be modified or adjusted to accommodate vehicle-specific information. 

Phase 2 GEM includes four main vehicle types, and several variations of each vehicle, to match 

the regulatory subcategories in the proposed Phase 2 rulemaking. Each regulatory subcategory is 

associated with specific vehicle parameters and technology options. 

The agencies predefined many key parameters, since those parameters are either hard to 

quantify due to lack of certified testing procedures or difficult to obtain due to proprietary 

barriers. Examples of these parameters include transmission shifting strategies, transmission 

gear mechanical efficiency, and transmission spin and pumping losses. The values selected for 

these parameters are a result of substantial testing by EPA, as well as confidential discussions 

with engine, chassis and component manufacturers. 

Each vehicle subcategory has a set of user-defined parameters. These parameters include 

vehicle technologies or component attributes that impact CO2 emissions and fuel consumption, 

but have the potential to vary across manufacturers. Parameters such as aerodynamic 

performance, vehicle weight, engine fuel map, transmission gear ratios, tire radius, or axle ratio 

can all be changed as inputs by the user. 

The sections to follow outline the regulatory vehicle subcategories that would use GEM 

for compliance and summarizes the user-defined and predefined model parameters applicable to 

each subcategory. 

Drive Cycles and Transient Correction Factor 

The proposed rulemaking also predefines three drive cycles including a transient cycle 

and two cruise speed cycles. The transient mode is defined by California Air Resources Board 

(CARB) in their Highway Heavy-Duty Diesel Transient (HHDDT) cycle. The cruise speed 

cycles are represented by two constant speed 65 mph and 55 mph cycles, each with varying road 

grade. A stationary “idle cycle” is also available to vocational vehicles, which evaluates both 

zero torque and curb-idle transmission torque. Each regulatory subcategory is given a specific 

set of drive cycle weightings. 

The agencies recognize the limitation of the steady state engine fuel map for transient 

simulation, and therefore are proposing a transient adjustment factor of 1.05 used only for 

transient cycle. This means that the simulated transient cycle GEM results (that are generated 

based on steady-state fuel maps) would be multiplied by 1.05 before being output from the GEM 

compliance tool. The higher output value would be the official GEM result. The detailed 

description and justification of this transient correction factor can be viewed in Chapter 4 of the 

draft RIA. 

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Tractor Vehicle Parameters 

The agencies are proposing a set of predefined modeling parameters to establish 

consistent combination tractor models from which tractor manufacturers can compare their 

vehicle improvements. GEM recognizes nine variations of combination tractors. Class 8 

tractors can have day or sleeper cab configurations with low, mid, or high roof heights. Class 7 

tractors are only available in a day cab configuration, but also have low, mid, or high roof 

options. Within GEM, high roof tractors are simulated as pulling a standard box trailer. Mid 

roof tractors and low roof tractors are simulated as pulling tank and flatbed trailers, respectively. 

The agencies are proposing that the standard box trailer for high roof tractor simulations also 

include a skirt, which is represented by the user-defined aerodynamic drag area input, CdA 

(described in the next section). 

Table 1, Table 2, and Table 3 summarize the predefined modeling parameters for Class 8 

sleeper cab, Class 8 day cab, and Class 7 tractors, respectively. All Class 8 tractors are modeled 

with five axles in a 6x4 configuration with two steer tires and eight drive tires. The Class 8 

payload is 19 tons. All Class 7 tractors are represented by a four axle tractor in a 4x2 

configuration with two steer tires and four drive tires. The Class 7 payload is 12.5 tons. Sleeper 

cab tractors are assigned drive cycle weightings that are more representative of long-haul driving 

with 86 percent at 65-MPH, 9 percent at 55-MPH and 5 percent transient. Drive cycle 

weightings for day cab tractors are more representative of short-haul driving with 64 percent at 

65-MPH, 17 percent at 55-MPH, 19 percent transient. 

Table 1 Class 8 Combination Tractor Sleeper Cab Predefined Modeling Parameters 

Regulatory Subcategory Class 8 Combination, Sleeper Cab 

Roof Height High Roof Mid Roof Low Roof 

Total Weight (kg) 31978 30277 30390 

Number of Axles 5 

Default Axle Configuration 6x4 

Payload (tons) 19 

CARB HHDDT Drive Cycle Weighting 0.05 

GEM 55 mph Drive Cycle Weighting 0.09 

GEM 65 mph Drive Cycle Weighting 0.86 

Table 2 Class 8 Combination Tractor Day Cab Predefined Modeling Parameters 

Regulatory Subcategory Class 8 Combination, Day Cab 

Roof Height High Roof Mid Roof Low Roof 

Total Weight (kg) 31297 29529 29710 

Number of Axles 5 

Default Axle Configuration 6x4 

Payload (tons) 19 

CARB HHDDT Drive Cycle Weighting 0.19 

GEM 55 mph Drive Cycle Weighting 0.17 

GEM 65 mph Drive Cycle Weighting 0.64 

Table 3 Class 7 Combination Tractor Predefined Modeling Parameters 

Regulatory Subcategory Class 7 Combination, Day Cab 

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Roof Height High Roof Mid Roof Low Roof 

Total Weight (kg) 22679 20910 21091 

Number of Axles 4 

Default Axle Configuration 4x2 

Payload (tons) 12.5 

CARB HHDDT Drive Cycle Weighting 0.19 

GEM 55 mph Drive Cycle Weighting 0.17 

GEM 65 mph Drive Cycle Weighting 0.64 

Table 4 shows the predefined modeling parameters that are consistent across all tractor 

types. These common parameters include ambient temperature, efficiencies and accessory 

powers. The calculations for overall rolling resistance and the distribution of weight savings are 

also consistent for all modeled tractors. 

Table 4 Common Predefined Modeling Parameters for All Simulated Combination Tractors 

Gearbox Efficiency 98% for 1:1 gear ratio, 96% for others 

Axle Mechanical Efficiency 95.5% 

Electrical Accessory Power (W) 300 

Mechanical Accessory Power (W) 1000 

Environmental air temperature (°C) 25 

Weight Reduction (lbs) Add 1/3*weight reduction to Payload tons 

Trailer Tire Crr (kg/t) 6.0 

Overall Tire Crr (kg/t) = 0.425*Trailer Crr + 0.425*Drive Crr + 0.15*Steer Crr 

GEM allows a user to modify or adjust performance information for certain components 

in order to model and quantify improvements the manufacturer is making to its vehicles. Table 5 

lists the user-defined modeling parameters that are recognized in GEM. Phase 2 GEM continues 

to allow tractor manufacturers to model their vehicle’s tire rolling resistances, but also requires 

each tire’s loaded radius. Phase 2 GEM also continues to account for aerodynamic drag, but 

manufacturers are asked to provide a wind-averaged aerodynamic drag area (CdA), instead of the 

zero-yaw coefficient of drag (Cd) required in Phase 1. These updates were made in order to 

more realistically model the rolling resistance and aerodynamic drag the vehicle would 

experience in real world driving. 

In Phase 1, a default engine and transmission were applied to all GEM-simulated 

combination tractors. Phase 2 GEM requires manufacturers to supply an engine fuel map for 

each distinct engine used in the modeled vehicles. Additionally, the manufacturer must include 

specific transmission information to the model. The engine and the transmission information are 

provided as separate input files with specific requirements, as will be discussed in the Input Files 

section of this guide. 

Table 5 User-Defined Modeling Parameters for Class 7 and Class 8 Combination Tractors (All Cabs and
 

Roof Heights)
 

Modeling Parameter Method of Determining Parameter 

Steer Tire Crr (kg/t) ISO 28580:2009(E), See 40 CFR 1037.520(c) 

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Drive Tire Crr (kg/t) 

Tire Loaded Radius (m) See 40 CFR 1037.520(c) 

Aerodynamic Drag Area, CdA (m2) See 40 CFR 1037.520(b) and 40 CFR 1037.525 

Engine Fuel Map From engine manufacturer or 40 CFR 1065.510 

Transmission From transmission manufacturer: type, gear number, gear ratios 

Drive Axle Ratio From axle manufacturer 

Phase 2 GEM also accounts for additional technologies and vehicle configurations that 

reduce CO2 and fuel consumption, but are not easily captured in the vehicle simulation. These 

reduction values range from 0.5 percent to 2.5 percent and are applied internally to GEM after 

the vehicle simulation has completed. Manufacturers would indicate if their vehicle includes any 

of the technology improvements listed in Table 6 by indicating yes or no (Y or N in the input 

file) and GEM would apply the appropriate emission reduction. 

Table 6 Technology Improvement Options for Tractor Manufacturers 

Technology Improvement Regulation Reference Reduction Value 

Vehicle Speed Limit See 40 CFR 1037.520(d) Varies 

Weight Reduction (lb) See 40 CFR 1037.520(e) Varies 

Single Drive Axle (Class 8 Tractors Only) See 40 CFR 1037.520(f)(1) 2.5% 

Part Time Single Drive Axle (Class 8 Tractors Only) See 40 CFR 1037.520(f)(1) 2.5% a 

Low Friction Axle Lubricant See 40 CFR 1037.520(f)(2) 0.5% 

Transmissions: AT, AMT, DCT See 40 CFR 1037.520(f)(3) 2.0% 

Predictive Cruise Control See 40 CFR 1037.520(f)(4) 2.0% 

High Efficiency A/C Compressor See 40 CFR 1037.520(f)(5) 0.5% 

Electric Engine Coolant and Power Steering Pumps See 40 CFR 1037.520(f)(6) 1.0% 

Automatic Tire Inflation System See 40 CFR 1037.520(f)(7) 1.0% 

Extended Idle Reduction (Sleeper Cabs Only) See 40 CFR 1037.520(f)(8) 5.0% 

Notes: 

a Part Time Single Drive Axle reduction is 2.5% over the 55 mph and 65 mph cycles and zero over the CARB 

HHDDT cycle 

Vocational Vehicle Parameters 

The agencies are proposing a set of predefined modeling parameters to establish 

consistent vocational vehicle models from which manufacturers can compare their vehicle 

improvements. GEM recognizes nine variations of vocational vehicles based on both vehicle 

weight class and duty cycle. Class 8 vocational vehicles are considered heavy heavy-duty 

(HHD). Classes 6 and 7 are medium heavy-duty (MHD), and Classes 2b-5 are considered light 

heavy-duty (LHD). As seen in Table 7, Table 8, and Table 9, the weight, number of axles, 

aerodynamic drag area and payload are the same for all of the vehicles within a weight class. 

Vehicles within each weight class are further categorized using three duty cycles (Regional, 

Multi-purpose and Urban) by varying the drive cycle weightings associated with each composite 

duty cycle. The assignment of vocational vehicles to subcategories is described in the 

regulations at 40 CFR 1037.510, and is based on a combination of vehicle attributes and 

driveline parameters. The Multi-purpose cycle is the default for vocational vehicles unless the 

16 
 

specified criteria indicate another cycle is more appropriate, or if the certifying manufacturer 

requests placement in a different subcategory. GEM does not automatically pre-process 

vocational vehicle inputs to assign vocational vehicles to subcategories, though GEM may be 

used as a tool to assist in deciding how the criteria of 1037.510 apply to your vehicles. For 

vocational vehicles with conventional powertrains and engines that are certified over both the 

FTP and SET, manufacturers will generally need to know the following information in order to 

decide whether the multi-purpose duty cycle is not the most appropriate for a vocational vehicle: 

tire revs/mile, rear axle ratio, transmission gear ratios, maximum engine test speed, and primary 

fuel type. 

Table 7 Vocational HHD Vehicle Predefined Modeling Parameters 

Regulatory Subcategory HHD 

Duty Cycle Regional Multi-Purpose Urban 

Total weight (kg) 19051 

Number of Axles 3 

CdA (m2) 6.86 

Payload (tons) 7.50 

ARB Transient Drive Cycle Weighting 0.50 0.82 0.94 

GEM 55 mph Drive Cycle Weighting 0.28 0.15 0.06 

GEM 65 mph Drive Cycle Weighting 0.22 0.03 0.00 

Idle Cycle Weighting 0.10 0.15 0.20 

Table 8 Vocational MHD Vehicle Predefined Modeling Parameters 

Regulatory Subcategory MHD MHD MHD 

Duty Cycle Regional Multi-Purpose Urban 

Total weight (kg) 11408 

Number of Axles 2 

CdA (m2) 5.40 

Payload (tons) 5.60 

ARB Transient Drive Cycle Weighting 0.50 0.82 0.94 

GEM 55 mph Drive Cycle Weighting 0.28 0.15 0.06 

GEM 65 mph Drive Cycle Weighting 0.22 0.03 0.00 

Idle Cycle Weighting 0.10 0.15 0.20 

Table 9 Vocational LHD Vehicle Predefined Modeling Parameters 

Regulatory Subcategory LHD LHD LHD 

Duty Cycle Regional Multi-Purpose Urban 

Total weight (kg) 7257 

Number of Axles 2 

CdA (m2) 5.40 

Payload (tons) 2.85 

ARB Transient Drive Cycle Weighting 0.50 0.82 0.94 

GEM 55 mph Drive Cycle Weighting 0.28 0.15 0.06 

GEM 65 mph Drive Cycle Weighting 0.22 0.03 0.00 

Idle Cycle Weighting 0.10 0.15 0.20 

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Table 10 shows the predefined modeling parameters that are consistent across all 

vocational vehicle types. These common parameters include ambient temperature, efficiencies 

and accessory powers. It should be noted that GEM will assign all automated manual or dual 

clutch transmissions a 98 percent efficiency for the gear with a 1:1 ratio and 96 percent 

efficiency for all others. All gears in an automatic transmission are assumed to have a 98 percent 

efficiency (pump losses and spin losses are accounted for separately from the gear losses for 

automatic transmissions). The calculations for overall rolling resistance and the distribution of 

weight savings are also consistent for all modeled vocational vehicles. 

Table 10 Common Predefined Modeling Parameters for All Vocational Vehicles 

Gearbox Efficiency for AMT 98% for 1:1 gear ratio, and 96% for others 

Gearbox Efficiency for AT 98% for all gears 

Axle Mechanical Efficiency 95.5% 

Electrical Accessory Power (W) 300 

Mechanical Accessory Power (W) 1000 

Environmental Air Temperature (°C degree) 25 

Weight Reduction (lbs) Add 0.5*weight reduction to Payload tons 

Overall Tire Crr (kg/t) = 0.7*Drive Crr + 0.3*Steer Crr 

GEM allows a user to modify or adjust performance information for certain components 

in order to model and quantify improvements the manufacturer is making to its vehicles. Table 

11 lists the user-defined modeling parameters that are recognized in GEM for vocational 

vehicles. In Phase 1 GEM, vocational vehicle manufacturers were only given the option of 

changing the rolling resistance of their steer and drive tires. Phase 2 GEM continues to allow 

vocational vehicle manufacturers to model their vehicle’s tire rolling resistances, but also 

requires each tire’s loaded radius. Additionally, Phase 2 GEM requires manufacturers to supply 

an engine fuel map and specific transmission information to for each distinct engine and 

transmission in the vehicles being modeled. The engine and the transmission information are 

provided as separate input files with specific requirements, as will be discussed in the Input Files 

section of this guide. Note that emergency vehicles are only allowed to adopt lower rolling 

resistance tires on their steer and drive tires in this proposal. The rest of the parameters do not 

apply to emergency vehicles. 

Table 11 User-Defined Modeling Parameters for Vocational Vehicles (All Weight Classes) 

Modeling Parameter Method of Determining Parameter 

Steer Tire Crr (kg/t) ISO 28580:2009(E), See 40 CFR 1037.520(c) Drive Tire Crr (kg/t) 

Tire Loaded Radius (m) See 40 CFR 1037.520(c) 

Engine Fuel Map From engine manufacturer or 40 CFR 1065.510 

Transmission From transmission manufacturer: type, gear number, gear ratios 

Drive Axle Ratio From axle manufacturer 

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As is proposed for the tractor model, Phase 2 GEM accounts for additional vocational 

vehicle technologies that reduce CO2 and fuel consumption, but are not easily captured in the 

vehicle simulation. These reduction values range from 0.5 percent to 2.5 percent and are applied 

internally to GEM after the vehicle simulation has completed. Manufacturers would indicate if 

their vehicle includes any of the technology improvements listed in Table 12 by indicating yes or 

no (Y or N in the input file) and GEM would apply the appropriate emission reduction. 

Table 12 Technology Improvement Options for Vocational Vehicle Manufacturers 

Technology Improvement Regulation Reference Reduction Value 

Weight Reduction (lb) See 40 CFR 1037.520(e) Varies 

Power Take Off (g/ton-mile) See 40 CFR 1037.540 Varies 

HHD Single Drive Axle (6x2) Configuration See 40 CFR 1037.520(f)(1) 2.5% 

HHD Part-time Single Drive Axle 6x2 Configuration See 40 CFR 1037.520(f)(1) 0.1% - 1.0%a 

Low Friction Axle Lubricant See 40 CFR 1037.520(f)(2) 0.5% 

Transmissions: AMT, DCT (HHD, Regional Only) See 40 CFR 1037.520(f)(3) 2.3% 

Idle Reduction: Start-Stop See 40 CFR 1037.520(f)(8) Determined by idle 

Idle Reduction: Neutral Idle See 40 CFR 1037.520(f)(8) fuel map 

Notes:
 a Based on 2.5% over the 55 mph and 65 mph cycles and zero over the CARB HHDDT cycle
 

Trailer Vehicle Parameters 

The agencies are proposing an equation-based compliance approach for box trailer 

manufacturers and they are not required to certify their trailers using GEM. However, the 

equations for each box trailer subcategory are based on the simulated trailers described in this 

section. Note that non-box trailers do not use GEM or the GEM-based equation for compliance 

and a discussion of non-box trailers is not included in this user guide. The following description 

of the trailer model as it applies to box trailers is included for informational purposes only. 

The agencies are proposing a set of predefined modeling parameters to establish 

consistent tractor-trailer models from which box van trailer manufacturers can compare their 

vehicle improvements. GEM recognizes four variations of box van based on length. All 

simulated box vans are modeled as a combination tractor-trailer with a Class 8 high roof tractor. 

Long box vans (trailers that are longer than 50-feet) are represented by either a 53-foot dry van 

or a 53-foot refrigerated van in GEM. The tractor-trailer model for long-box vans includes a 

sleeper cab tractor and is given the same long-haul drive cycle weightings as the Class 8 high 

roof sleeper cab tractors mentioned previously. GEM models all short box vans (box trailers 50­

feet in length and shorter) as a single-axle, solo 28-foot dry van or refrigerated van. 

Table 13 and Table 14 summarize the predefined modeling parameters for long and short 

box vans, respectively. All long box vans are modeled with tandem axles and a payload of 19 

tons and drive cycle weightings that are more representative of long-haul driving (i.e., 86 percent 

at 65-MPH, 9 percent at 55-MPH and 5 percent transient). All short box vans are modeled with 

a single axle, a payload of 10 tons, and drive cycle weightings more representative of short-haul 

driving with 64 percent at 65-MPH, 17 percent at 55-MPH, 19 percent transient. The vehicle 

weight varies between dry vans and refrigerated vans to account for the weight of the 

19 
 

 

 

 

 

refrigeration unit, and weight also varies proportional to the length of the trailer. The baseline 

CdA values were obtained from EPA’s aerodynamic testing. 

Table 13 Predefined Modeling Parameters for Long Box Trailers 

Regulatory Subcategory Long Box 

Dry Van 

Long Box 

Refrigerated Van 

Tractor Type C8 Sleeper Cab - High Roof 

Total weight (kg) 31978 33778 

Baseline CdA Values (m2) 6.2 6.1 

Number of Axles 5 

Payload (tons) 19 

CARB HHDDT Drive Cycle Weighting 0.05 

GEM 55 mph Drive Cycle Weighting 0.09 

GEM 65 mph Drive Cycle Weighting 0.86 

Table 14 Predefined Modeling Parameters for Short Box Trailers 

Regulatory Subcategory Short Box 

Dry Van 

Short Box 

Refrigerated Van 

Tractor Type C8 Day Cab - High Roof 

Total weight (kg) 15191 16991 

Baseline CdA Values (m2) 6.1 6.0 

Number of Axles 4 

Payload (tons) 10 

ARB Transient Drive Cycle Weighting 0.19 

GEM 55 mph Drive Cycle Weighting 0.17 

GEM 65 mph Drive Cycle Weighting 0.64 

Table 15 shows the predefined modeling parameters that are consistent across all trailer 

types. Many of these common parameters are associated with the simulated tractor in the tractortrailer model. The calculations for overall rolling resistance and the distribution of weight 

savings are consistent with the calculations for GEM-simulated tractors. 

Table 15 Common Predefined Modeling Parameters for All Box Trailers 

Engine Fuel Map 15L - 455 HP 

Gear ratio 12.8, 9.25, 6.76, 4.9, 3.58, 2.61, 1.89, 1.38, 1, 0.73 

Gearbox Efficiency 98% for 1:1 gear ratio, and 96 for all other gears 

Axle Drive Ratio 3.7 

Axle Mechanical Efficiency 95.5% 

Default Tractor Axle Configuration 6x4 

Electrical Accessory Power (W) 300 

Mechanical Accessory Power (W) 1000 

Tire Radius (m) 0.5 

Steer Tire Crr (kg/t) 6.54 

Drive Tire Crr (kg/t) 6.92 

Overall Tire Crr (kg/t) = 0.425*Trailer Crr + 0.425*Drive Crr + 0.15*Steer Crr 

Weight Reduction (lbs) Add 1/3*weight reduction to Payload tons 

20 
 

 

 

 

 

 

 

 

 

 

GEM allows a user to modify or adjust performance information for certain components 

in order to model and quantify improvements the manufacturer is making to its vehicles. The 

trailer program has three user-defined parameters and one pre-defined technology improvement 

option that has a specified reduction value associated with its use (see Table 16). Trailer 

manufacturers are able to change their tire rolling resistance, aerodynamic drag and cumulative 

weight reduction. GEM applies an additional technology improvement value of 1.5 percent for 

manufacturers that indicate they installed automatic tire inflation systems on their simulated 

trailer. 

Table 16 User-Defined Modeling Parameters and Technology Improvement Options for Trailers (All
 

Lengths)
 

Modeling Parameter Method of Determining Parameter 

Trailer Tire Crr (kg/t) ISO 28580:2009(E), See 40 CFR 1037.515(b) 

Change in Aerodynamic Drag Area, ∆CdA (m2) See 40 CFR 1037.515(c) and 40 CFR 1037.525 

Weight Reduction (lb) See 40 CFR 1037.515(d) 

Automatic Tire Inflation System 1.5% if applied, See 40 CFR 1037.515(a) 

GEM Input File Structure 

As mentioned previously, Phase 2 GEM does not offer a graphical user interface (GUI) 

for users to provide their vehicle parameters. Instead, inputs are exclusively provided to the 

model in a .csv file. Phase 2 GEM can be executed using either of two methods. The standard 

method directly initiates the model by running the executable file. Inputs for the standard 

method are provided as .csv files. The second method requires use of Microsoft Excel. Users 

generate an .xlsm input file using the template provided for their given vehicle subcategory and 

initiate the model by clicking the “RUN GEM” button within the spreadsheet. The button 

prompts a macro to generate the appropriate .csv file from the spreadsheet provided, and runs the 

executable. Additionally, the Excel spreadsheet features a “Validate” button which will check 

the input file for errors without running the executable. All methods provide their results in a 

.csv output file and an error log .txt file. 

Some manufacturers that choose the standard method of running Phase 2 GEM may 

create a script to automatically generate their input files in .csv format. Others may wish to 

manually populate their files using a spreadsheet tool, such as Microsoft Excel, to easily view the 

input fields in a column format, and save the files in a .csv format. Some manufacturers may opt 

for the Excel-based method of generating inputs and creating the .csv input files through a 

macro. The following subsections describe the input file structures, which are consistent for 

each method of running the model. For illustration purposes, the .csv input files in this section 

are shown in spreadsheet format. The following section, Running GEM, describes the options 

for running the model. 

21 
 

 

 

 

Note to Users Creating or Editing .csv Files Using Microsoft Excel 

Users may use Microsoft Excel or any other text editor, such as Notepad, to create or edit 

their input files. Once a file is created, it can be saved by going to go to “Save As...” and choose 

CSV (Comma delimited) (*.csv) in the “Save as type:” pull-down menu, as shown in Figure 12. 

In future saves, or when editing and resaving an existing .csv formatted file, users may receive a 

warning similar to Figure 13. Select “Yes” to ensure the file is properly saved in .csv format. 

Figure 12: Example "Save As..." Window to Create a Comma Separated Value (.csv) Input File 

Figure 13: Warning from Microsoft Excel When Saving .csv Files Using Excel – Select “Yes” 

Tractor Input Files 

The top of the tractor input file has three lines that list the regulatory category, 

manufacturer name and model year (see Figure 14). Users complete the second column for each 

line. The first line must read “Tractor” in order for GEM to run the appropriate vehicle model. 

Manufacturer name can be in any format, but it should be consistent with other regulatory 

documents from the manufacturer. Model year should be expressed as a four-digit number. 

22 
 

 

 

 

 

 

 

Figure 14: Tractor Input File Header Information 

The next lines of the tractor input file contain the model inputs. In the first two columns, 

shown in Figure 15, the user provides a Run ID and the Regulatory Subcategory for each run. 

The Run ID is a unique value that will be used to identify the run (e.g., vehicle VIN). It can be 

any combination of letters, numbers and separators such as dash (“-“), periods (“.”), or 

underscores (“_”). The Regulatory Subcategory determines which standard is applied for 

compliance. For tractors, there are 10 regulatory subcategories in GEM and Table 17 shows the 

naming convention. 

Figure 15: Tractor Input File Run ID and Regulatory Subcategory Inputs
 

Table 17: Tractor Input File Naming Convention for Tractor Regulatory Subcategories
 

GEM Input Name Regulatory Subcategory Description 

C8_SC_HR Class 8 Combination, Sleeper Cab - High Roof 

C8_SC_MR Class 8 Combination, Sleeper Cab - Mid Roof 

C8_SC_LR Class 8 Combination, Sleeper Cab - Low Roof 

C8_DC_HR Class 8 Combination, Day Cab - High Roof 

C8_DC_MR Class 8 Combination, Day Cab - Mid Roof 

C8_DC_LR Class 8 Combination, Day Cab - Low Roof 

C8_HH Class 8 Combination, Sleeper Cab - Heavy Haul 

C7_DC_HR Class 7 Combination, Day Cab - High Roof 

C7_DC_MR Class 7 Combination, Day Cab - Mid Roof 

C7_DC_LR Class 7 Combination, Day Cab - Low Roof 

The next columns contain the tractor performance parameters and several vehicle 

characteristics that are user-defined in GEM. A description of these parameters was given in 

Table 5 and additional information is available preamble to the proposed Phase 2 rulemaking. 

23 
 

 

 

 

Figure 16: Tractor Input File Performance Parameters and User-Defined Vehicle Characteristics 

There are limits associated with each user-defined input value. Table 18 shows the limits 

for the five tractor inputs in the model. GEM will produce an error if any of these values are out 

of the acceptable range. 

Table 18: Minimum and Maximum Limits for User-Defined Values in Tractor Input File 

User-Defined Parameter Units Minimum 

Value 

Maximum 

Value 

Aerodynamic Drag Area (CdA) m^2 4.1 7.6 

Tire Radius m 0.3 0.6 

Steer Axle Tire, Crr kg/t 4.0 8.1 

Drive Axle Tire, Crr kg/t 4.2 8.5 

Drive Axle Ratio # 1.8 10 

The next columns in the input file are for the engine and transmission file names or the 

powertrain file name. A description of the content of these supplemental input files is located at 

the end of this section. The text in the fields of the input file must exactly match the file name, 

including the .csv extension for the code to run properly. In Figure 17, these supplemental files 

are located in the same folder as the tractor input file. As a result, only the file name is needed. 

Manufacturers may choose to perform powertrain testing to obtain the engine and 

transmission performance for their GEM simulations. In order to indicate to GEM that 

powertrain data is provided, users would provide the same powertrain input file name in both the 

Engine and Transmission file name fields of the tractor input file, as seen in the last three rows of 

Figure 17 

Figure 17: Tractor Input File Reference to Engine, Transmission, and Powertrain Input Files 

24 
 

Users can also save their engine, transmission, and powertrain input files in separate 

folders within the same folder as the main vehicle input file, as shown in Figure 18. At this time, 

the model is not able to reference folders for the supplemental input files located outside of the 

vehicle input file’s directory. 

Figure 18: Option to Save Engine, Powertrain, and Transmission Input Files in Separate Folders 

If the supplemental input files are located in the folders shown in Figure 18, the text in 

the Engine and Transmission fields of the input file would match the text in Figure 19. 

Figure 19: Tractor Input File Reference to Engine, Transmission, and Powertrain Input Files When Files are 

Located in Separate Folders 

The next columns in the tractor input file are for the optional technology improvements. 

These technology improvements were summarized in Table 6. These technology improvement 

fields cannot be blank in the input file. Users that wish to model their vehicle with weight 

reduction can enter the cumulative weight reduction value (0 to 10,000 lbs or “NA”) in the first 

field. The use of a speed limiter is modeled by inserting the speed limit value. Note that GEM 

does not provide a benefit for speed limiters set above 65 mph. Vehicles without a speed limiter 

would use the value “NA”. The remaining technology improvements are Boolean values (“Y” 

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for yes, “N” for no). By default, these inputs are set to “N”. Note that the Single Drive Axle 

improvement is only available to Class 8 tractors, since Class 7 tractors are only modeled with a 

single-drive axle, and that Idle Reduction with APU is only available to the Class 8 sleeper cab 

tractors. Also note that these inputs are case-sensitive. All values of “Y”, “N”, or “NA” must be 

in UPPERCASE LETTERS. Lowercase letters will produce an error. 

Table 19: Summary of Technology Improvements in Tractor Input File 

Technology Improvement Units Range 

Weight Reduction lbs 0 to 10,000 or “NA” 

Speed Limiter MPH 0 to 65 or “NA” 

Single Drive Axle (applies to Class 8 only) Y/N Must be "Y" or "N", not blank 

Part Time Single Drive Axle (applies to Class 8 only) Y/N Must be "Y" or "N", not blank 

Low Friction Axle Lubricant Y/N Must be "Y" or "N", not blank 

Predictive Cruise Control Y/N Must be "Y" or "N", not blank 

High Efficiency A/C Compressor Y/N Must be "Y" or "N", not blank 

Electric Accessories Y/N Must be "Y" or "N", not blank 

Extended Idle Reduction (Class 8 sleeper cab tractors only) Y/N Must be "Y" or "N", not blank 

Automatic Tire Inflation System Y/N Must be "Y" or "N", not blank 

Users may note that Table 19 does not include the technology improvement input from 

automated transmissions as indicated from Chapter 4 of the draft RIA. This is because GEM is 

designed in such a way that once users select automated manual, dual clutch or automatic 

transmission in the transmission input .csv file, the technology improvement input is 

automatically added into the post processor to get benefits. 

Vocational Input Files 

The top of the vocational input file has three lines that list the regulatory category, 

manufacturer name and model year (see Figure 20). The first line must read “Vocational” in 

order for GEM to run the appropriate vehicle model. Manufacturer name can be in any format, 

but it should be consistent with other regulatory documents from the manufacturer. Model year 

should be expressed as a four-digit number. 

Figure 20: Vocational Input File Header Information 

The next lines of the vocational input file contain the model inputs. In the first two 

columns, shown in Figure 21, the user provides a Run ID and the Regulatory Subcategory for 

each run. The Run ID is a unique value that will be used to identify the run (e.g., vehicle VIN). 

It can be any combination of letters, numbers and separators such as dash (“-“), periods (“.”), or 

underscores (“_”). The Regulatory Subcategory determines which standard is applied for 

26 
 

 

compliance. For vocational, there are 12 regulatory subcategories in GEM and Table 20 shows 

the naming convention. 

Figure 21: Vocational Input File Run ID and Regulatory Subcategory Inputs
 

Table 20: Vocational Input File Naming Convention for Vocational Regulatory Subcategories
 

GEM Input Name Regulatory Subcategory Description 

HHD_R Heavy-Heavy Duty, Regional 

HHD_M Heavy-Heavy Duty, Multipurpose 

HHD_U Heavy-Heavy Duty, Urban 

MHD_R Medium-Heavy Duty, Regional 

MHD_M Medium-Heavy Duty, Multipurpose 

MHD_U Medium-Heavy Duty, Urban 

LHD_R Light-Heavy Duty, Regional 

LHD_M Light-Heavy Duty, Multipurpose 

LHD_U Light-Heavy Duty, Urban 

HHD_E Heavy-Heavy Duty, Emergency 

MHD_E Medium-Heavy Duty, Emergency 

LHD_E Light-Heavy Duty, Emergency 

The next columns contain the vocational performance parameters and vehicle 

characteristics that are user-defined in GEM. A description of these parameters was given in 

Table 12 and additional information is available in the preamble to the proposed Phase 2 

rulemaking. 

Figure 22: Vocational Input File Performance Parameters and User-Defined Vehicle Characteristics 

27 
 

 

 

 

 

There are limits associated with each user-defined input value. Table 21 shows the limits 

for the four vocational inputs in the model. GEM will produce an error if any of these values are 

out of the acceptable range. 

Table 21: Minimum and Maximum Limits for User-Defined Values in Vocational Input File 

User-Defined Parameter Units Minimum 

Value 

Maximum 

Value 

Tire Radius m 0.2 0.7 

Steer Axle Tire Crr kg/t 4.0 12.0 

Drive Axle Tire Crr kg/t 4.0 12.0 

Drive Axle Ratio none 1.8 12.0 

The next columns in the input file are for the engine and transmission file names or the 

powertrain file name. A description of the content of these supplemental input files is located at 

the end of this section. The text in the fields of the input file must exactly match the file name, 

including the .csv extension for the code to run properly. In Figure 23, these engine or 

transmission files are located in the same folder as the vocational input file. As a result, only the 

file name is needed. 

Manufacturers may choose to perform powertrain testing to obtain the engine and 

transmission performance for their GEM simulations. In order to indicate to GEM that 

powertrain data is provided, users would provide the same powertrain input file name in both the 

Engine and Transmission file name fields of the tractor input file, as seen in the last three 

columns of Figure 23. 

Figure 23: Vocational Input File Reference to Engine, Transmission, and Powertrain Input Files 

Users can also save their engine, transmission, and powertrain input files in separate 

folders within the same folder as the main vehicle input file, as shown in Figure 24. 

28 
 

 

 

 

 

 

 

 

 

Figure 24: Option to Save Engine, Powertrain, and Transmission Input Files in Separate Folders 

If the supplemental input files are located in the folders shown in Figure 24, the text in 

the Engine and Transmission fields of the vocational input file would match the text in Figure 

25. Note that the fifth row contains “NA” for both the engine and transmission file names, since 

the vehicle modeled in the sample file is an emergency vehicle. 

Figure 25: Vocational Input File Reference to Engine, Transmission, and Powertrain Input Files When Files 

are Located in Separate Folders 

The next columns in the vocational input file are for the optional technology 

improvements. These technology improvements were summarized in Table 11. These 

technology improvement fields cannot be blank in the input file. Users that wish to model their 

vehicle with weight reduction can enter the cumulative weight reduction value (0 to 10,000 lbs or 

“NA”) in the first field. The remaining technology improvements are Boolean values (“Y” for 

yes, “N” for no). By default, these inputs are set to “N”. Note that the Single Drive Axle 

improvement (also known as axle disconnect) is only available to Heavy Heavy-Duty Regional 

vehicles. If selected for other vocational subcategories, the input will produce an error. Also, 

note that the stop-start and neutral idle technologies are mutually exclusive and users will receive 

an error if both technology options are selected for the same vehicle. If the vocational vehicle 

will be built with a hybrid PTO and testing was conducted according to 40 CFR 1037.540, then 

the Delta PTO value obtained from that test procedure may be entered. Also note that these 

29 
 

 

 

 

 

 

 

 

 

 

 

inputs are case-sensitive. All values of “Y”, “N”, or “NA” must be in UPPERCASE LETTERS. 

Lowercase letters will produce an error. 

Table 22: Summary of Technology Improvements in Vocational Input File 

Technology Improvement 

Delta PTO Fuel g/ton-mile 0 ... 30 or “NA” 

Weight Reduction lbs 0 ... 10,000 or “NA” 

Single Drive Axle (applies to HHD only) Y/N Must be "Y" or "N", not blank 

Part Time Single Drive Axle (applies to HHD only) Y/N Must be "Y" or "N", not blank 

Low Friction Axle Lubricant Y/N Must be "Y" or "N", not blank 

Neutral-Idle Y/N Must be "Y" or "N", not blank 

Stop-Start Y/N Must be "Y" or "N", not blank 

Similar to tractors, Table 22 does not include the technology improvement inputs for 

automated transmissions for HHD Regional vehicles, once users select an automated manual or 

dual clutch transmission in the transmission input .csv file for HHD Regional vehicles, the 

technology improvement input is automatically added into the post processor to get benefits. For 

all subcategories, GEM simulates DCT as AMT. 

Trailer Input Files 

The top of the trailer input file has three lines that list the regulatory category, 

manufacturer name and model year (see Figure 26). The first line must read “Trailer” in order 

for GEM to run the appropriate vehicle model. Manufacturer name can be in any format, but it 

should be consistent with other regulatory documents from the manufacturer. Model year should 

be expressed as a four-digit number. 

Figure 26: Trailer Input File Header Information 

The next lines of the trailer input file contain the model inputs. In the first two columns, 

shown in Figure 27, the user provides a Run ID and the Regulatory Subcategory for each run. 

The Run ID is a unique value that will be used to identify the run. It can be any combination of 

letters, numbers and separators such as dash (“-“), periods (“.”), or underscores (“_”). The 

Regulatory Subcategory determines which standard is applied for compliance. For trailers, there 

are four regulatory subcategories modeled in GEM and Table 23 shows the naming convention. 

30 
 

 

 

 

Figure 27: Trailer Input File Run ID and Regulatory Subcategory Inputs
 

Table 23: Trailer Input File Naming Convention for Trailer Regulatory Subcategory Inputs
 

GEM Input Name Regulatory Subcategory Description 

LDV Long Dry Van 

LRV Long Refrigerated Van 

SDV Short Dry Van 

SRV Short Refrigerated Van 

The next columns contain the trailer performance parameters and technology 

improvement options that are user-defined in GEM. A description of these parameters was given 

in Table 16 and additional information is available preamble to the proposed Phase 2 

rulemaking. Note that the aerodynamic improvement for trailers is measured as a delta CdA and 

not the absolute CdA value used in the proposed tractor program. 

Figure 28: Trailer Input File Performance Parameters and User-Defined Vehicle Characteristics 

There are limits associated with each user-defined input value. Table 21 shows the limits 

for the two trailer inputs in the model. GEM will produce an error if any of these values are out 

of the acceptable range. 

Table 24: Minimum and Maximum Limits for User-Defined Values in Trailer Input File 

User-Defined Parameter Units Minimum 

Value 

Maximum 

Value 

Aerodynamic Drag Area (CdA) m^2 0 2 

Tire Rolling Resistance Level (TRRL) kg/t 3 7 

The technology improvement fields cannot be blank in the input file. Users that wish to 

model their vehicle with weight reduction can enter the cumulative weight reduction value (0 to 

10,000 lbs or “NA”) in the first field. The automatic tire inflation input is a Boolean (“Y” for 

yes, “N” for no). By default, this inputs is set to “N”. Note that these inputs are case-sensitive. 

31 
 

 

 

 

 

 

 

 

All values of “Y”, “N”, or “NA” must be in UPPERCASE LETTERS. Lowercase letters will 

produce an error. 

Table 25: Summary of Technology Improvements in Trailer Input File 

Technology Improvement Units Range 

Weight Reduction lbs 0 to 10,000 or “NA” 

Automatic Tire Inflation System none Must be "Y" or "N", not blank 

Supplemental Input Files 

Tractor and vocational vehicle manufacturers are required to generate either separate 

engine and transmission input files, or a single powertrain input file for GEM. These files must 

be in .csv format to be properly read by the model. The GEM installation file includes a 

template for these supplemental input files. A vehicle manufacturer would generate a separate 

engine and transmission file for each engine and transmission used in its vehicles or separate 

powertrain files for each engine and transmission combination tested. We recommend that the 

manufacturers choose a consistent naming convention that provides unique file names for these 

input files. An example naming convention for each file type is provided in Table 26. However, 

manufacturers are free to choose their own naming convention for all of their input files. 

Table 26 Example Naming Convention for GEM Input Files 

Input File Type Example Naming Convention 

Engine MfrName_FuelType_ModelYear_ModelName_CalibrationID.csv 

Transmission MfrName_TransType_Gears_ModelName_CalibrationID.csv 

Powertrain MfrName_EngineName_TransName_ModelYear_CalibrationID.csv 

Engine Input File for Tractor and Vocational Vehicles 

The first row of the GEM input file for engines shows the GEM version, and 

manufacturers would fill in the fourth row with the Manufacturer Name, Combustion Type, Fuel 

Type, Family Name, and Calibration ID, as shown in Figure 29. The manufacturer can choose 

any Manufacturer Name, Family Name and Calibration ID, but the names should be consistent 

with other regulatory documents from the manufacturer. Combustion Type and Fuel Type have 

specific options for a manufacturer to choose. 

Figure 29: Sample Engine Input File Header Information 

The manufacturer then specifies the single engine idle speed point (in RPM) at curb idle 

transmission torque (CITT), the engine full-load torque curve, and the engine motoring torque 

curve. Torque curves are specified in RPM and Nm, and procedures for producing the torque 

32 
 

 

 

curves, including the number of speed and torque points to measure, are specified in 40 CFR 

1065.510. Finally the manufacturer provides the four-point engine idle fuel map and the 143­

point engine fuel map. The speed, torque, and fuel rate in the fuel maps are specified in RPM, 

Nm, and g/s, respectively. Procedures for generating the idle and full engine fuel maps are 

specified in 40 CFR 1036.535 and 1065.510. Figure 30 shows a summary of the sample engine 

input file included with Phase 2 GEM. 

Figure 30: Sample Engine Input File Engine Characteristics 

33 
 

Transmission Input File for Tractor and Vocational Vehicles 

The first row of the GEM input file for transmissions shows the GEM version, and 

manufacturers would fill in the fifth row with the transmission’s Manufacturer Name, Type, and 

Model Name (see Figure 31). The vehicle manufacturer can choose any Manufacturer Name and 

Model Name for the transmission, but it should remain consistent with other regulatory 

documents. The transmission Type has specific options for the manufacturer to choose. 

Figure 31: Sample Transmission Input File Header Information 

Vehicle manufacturers would provide the gear ratios for their specific transmission, as 

seen in the sample shown in Figure 32. If the transmission is an automatic transmission that has 

a lockup gear other than the one programmed in the GEM shift strategy, the benefits of this will 

not be recognized unless a powertrain test is conducted. 

Figure 32: Sample Transmission Input File Transmission Gear Ratio Information 

Optional Powertrain Input File for Tractor and Vocational Vehicles 

Users may opt to use engine and transmission performance data obtained from a 

powertrain test in their GEM runs. Procedures for generating the data to populate a powertrain 

input file can be found in 40 CFR 1037.550. This section summarizes the input file format. 

The first row of the GEM input file from powertrain testing show the GEM version and 

manufacturers would fill in the fifth, ninth, and thirteenth rows with the relevant information 

about the engine, transmission and powertrain, respectively. Figure 33 shows the header 

information required by the model. 

34 
Figure 33: Sample Powertrain Input File Header Information 

The next rows of the powertrain input file, as seen in Figure 34, include the engine idle 

fuel rate and the powertrain performance information from the 55-MPH, 65-MPH, and Transient 

drive cycle tests. Users would supply the cycle work, output speed/vehicle speed, and fuel mass 

according to the procedures outlined in 40 CFR 1037.550(o). 

Figure 34: Sample Powertrain Input File Performance Information 

35 
 

 

 

 

 

 

 

 

GEM Output File Structure 

An output file will be generated when the vehicle simulation completes and it is 

automatically saved to the same location as the input file. As will be described in the following 

section, the output file naming convention depends on the method chosen to run GEM. If users 

run GEM from the Start Menu or command prompt, the output file name will be given the same 

name as the input file with a “_result.csv” extension. If users choose to run GEM using the 

Excel macro template, the output file will be renamed to match the date and time of the 

simulation, in the format: GEM_YYYYMMDD_HHmmSS_result.csv. During the simulation 

using either method, an error log file will be generated, even if there are no errors, with the same 

base name as the output file, except that the “_result.csv” extension would be replaced with a 

“_errors.txt” extension. 

Each output file is identical to the input file for the vehicle modeled, except that the last 

columns of the .csv file are populated with the model’s results. Figure 35 shows the results of an 

example simulation. The results fields are the same for each vehicle type, but their column 

location in the file varies. The first result column indicates the date and time when the 

simulation was performed. The next two columns are the raw GEM CO2 emissions and fuel 

consumption values. The next two columns are the Family Emissions Limit (FEL) values EPA 

and NHTSA will use for compliance. EPA’s CO2 results are integer values. NHTSA’s fuel 

consumption results are calculated from those CO2 integer values, and are reported with four 

decimal place precision to ensure consistency between the agencies’ results. 

Figure 35: Example Results Columns in GEM Output File 

The final three columns of the output file are left blank by the model. Manufacturers 

would manually add the appropriate subfamily name, target FEL for the given subfamily, and the 

volume of vehicles that will use the resulting FEL value prior to submitting their output file for 

compliance. 

Running GEM 

There are three options for running GEM. The first option directly accesses the program’s 

executable file via the Start Menu folders, the Start Menu search, or a GEM icon on the 

computer’s desktop screen. This first option initiates the GEM graphical user interface (GUI) to 

select and run the model. The second option uses the command prompt, which can also initiate 

the GEM GUI, but has the option to suppress it. The third option uses a Microsoft Excel macro 

to convert a spreadsheet template into the appropriate input files and run the executable with the 

GEM GUI. Each of these options is described in the following sections. 

36 
 

 

 

 

 

 

 

 

 

 

Preparing for GEM Runs 

Prior to running GEM, it is important to locate all of the necessary input files. Many 

computers will produce warnings if the users try to make changes to files in the installation 

folder if it is located in the C:\Program Files directory. It is recommended that users save a copy 

of the input file templates from the installation folder (i.e., C:\Program Files\US EPA\Phase 2 

GEM\Sample Input Files – RELOCATE BEFORE USE) to a location on their hard drive (e.g., 

C:\Documents and Settings\UserName\My Documents). These files can then be changed as 

needed to reflect the users’ specific vehicles. See the previous sections for a description of the 

input files. 

Special Instructions for Heavy-Haul Tractor Manufacturers 

Tractors that qualify for the heavy-haul regulatory subcategory designation have special 

instructions for their vehicle input file. These tractors are not required to perform aerodynamic 

testing to obtain an aerodynamic drag area, CdA, for their GEM input. As a result, users are 

instructed to use a CdA value of “NA” for all heavy-haul tractors. This value is added to the 

third column of the tractor vehicle input file. All other fields should be populated using the same 

instructions provided previously for tractors. 

Special Instructions for Vocational Emergency Vehicle Manufacturers 

Vocational vehicles that qualify for the emergency vehicle regulatory subcategory 

designation also have special instructions for their vehicle input file. These manufacturers are 

only allowed to apply lower rolling resistance tires to their steer and drive tires. They do not 

have to provide engine or transmission information, and do not have the option of applying 

technology inputs. If the Regulatory Subcategory field in the vehicle input file is populated with 

an emergency vehicle (i.e., HHD_E, MHD_E, or LHD_E), users must fill-in the remaining fields 

in the file with “NA” or “N” as appropriate. See the sample input file for vocational vehicles for 

an example. 

Accessing Output Files 

Output files from the GEM simulations are automatically saved in the same location as 

the input files. They can be identified by their “_result.csv” or “_errors.txt” file extensions. 

Identical output files are generated for each of the methods described below. 

Running GEM Using the Start Menu and Desktop Icon 

Users can access the Phase 2 GEM executable from the Start Menu. Go to Programs > 

EPA Phase 2 GEM > Phase 2 GEM as shown in Figure 36. 

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Figure 36: How to Access Phase 2 GEM from the Start Menu 

Alternatively, users can access the executable by typing “Phase 2 GEM” in the Search 

programs and files box located at the bottom of the Start Menu, and hitting “Enter”. If users 

select the option – “Create Desktop Icon” during installation, users can double click the icon 

(shown previously in Figure 6) to run GEM as well to initiate the program. 

Once the executable is selected, a window will ask the user to select an input file, as 

shown in Figure 37. By default, the program will first look in the installation folder. As 

mentioned previously, all of the input files are in a separate location to avoid permissions 

warnings, and the necessary transmission and engine input files are located in the same directory 

as the input file to be run. In this example, we are storing our files on the Desktop. Select the 

appropriate input file (GEM_Sample_TRACTOR.csv in Figure 38) to begin the simulation. 

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Figure 37: Popup Window to Select an Input File Using the Start Menu Option; Defaults to Installation
 

Folder
 

Figure 38: Location of Input Files for Example GEM Simulations 

Once the input file is selected, the program will begin to run. A status window will step 

through each configuration as it runs, as shown in Figure 39. At any point in the simulation, 

users can stop the model by clicking the red “X” at the top right corner of the status window. A 

new window will appear that asks “Cancel current simulation?” If the user chooses “Yes”, it will 

stop execution (once the current simulation has completed, which may take a few seconds) and 

produce an output file with only the configurations that completed. 

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Figure 39: Sample Status Window Showing Progress of GEM Simulations 

The status window will indicate when the simulations are complete with a window 

similar to Figure 40. The resulting output file will be saved in the same location as the input file, 

and will be given the same name as the input file with a “_result.csv” extension added to the end 

(“GEM_Sample_TRACTOR_result.csv” in this example). When the simulation is complete, 

users can close the status window by clicking the red “X” in the top right corner of the window. 

Figure 40: Sample Status Window Showing Complete GEM Simulations 

This first example simulation completed without any errors. No errors were produced in 

the status window or the error log, shown in Figure 41, provides confirmation. 

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Figure 41: Sample Error Log with No Errors from Simulation 

If an error occurs in any simulations, the window will indicate the configurations that 

failed and the model will continue to the next simulation. In the example of Figure 42, Run ID 

“TRAC1” and “TRAC2” failed. The runs with errors are displayed in the status window and 

indicated in the resulting output file, as seen in Figure 43 and Figure 44. Additionally, GEM will 

produce a separate file, shown in Figure 45, with an extension of “_errors.txt” that lists details 

about any errors in a set of simulations. 

Figure 42: Sample Status Window Showing Progress of GEM Simulations with Failed Runs 

Figure 43: Sample Status Window Showing Complete and Failed Simulations 

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Figure 44: Example Output File with Two Errors 

Figure 45: Example Error Log Resulting from Failed Configurations 

Running GEM Using the Command Prompt 

Users can also initiate GEM using the command prompt. As seen in Figure 46, the 

command prompt can be found using the Start Menu’s Search programs and files feature. Once 

the executable cmd.exe is selected, a command window similar to the one shown in Figure 47 

will display. 

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Figure 46: Initiating the Command Prompt from the Start Menu Search Bar 

Figure 47: Windows Command Prompt Window 

Users can use the command line to run GEM using the GUI by typing the location and 

name of the GEM executable as shown in Figure 48. Note that Windows requires quotation 

marks (“ ”) surrounding folder and file names that contain spaces. Users would then be 

prompted to select the input file as seen previously in Figure 37 and be shown the progress of the 

code as shown in Figure 39 and Figure 40. 

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Figure 48: Command Prompt to Initiate GEM with the GUI Option 

Alternatively, users have the option of bypassing use of the GUI by including the location 

and name of the input file, and adding “-c” to the end of the command, as shown in Figure 49. In 

this example, the executable is stored in the default Program Files location and the input files are 

located in a folder on the desktop. Also seen in Figure 49, the output generally shown in the 

GEM status window is displayed in the command window instead. 

Figure 49: Command Prompt to Initiate GEM and Directly Apply Input Files (no GUI Option) 

Similar to the Start Menu method, the command window will display errors if they occur 

in the simulation, as shown in Figure 50. Also, an identical error log file as was shown in Figure 

45 is generated to show the specifics of the errors. 

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Figure 50: Command Prompt Display When an Input File Contains Errors 

Users can also proactively check if their output file contains errors by including “-t” in 

their command to initiate GEM, as shown in Figure 51. This option will not run this simulation. 

Instead, it simply warns the user which runs contain errors. The validation information is 

repeated in the “_results.csv” file that is generated, as shown in Figure 52. An error log (with 

extension “_errors.txt”) is also created to provide more specific information about the errors. 

Figure 51: Command Prompt to Check Errors Prior to Running GEM 

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Figure 52: Resulting Output File from GEM Input Validation 

Running GEM from Microsoft Excel 

GEM can also be initiated using the Microsoft Excel templates and their embedded 

macros. This option requires more user-interaction, but does have some helpful features to catch 

errors before running the model. The template files that include the GEM macro are saved for 

each vehicle category as “Microsoft Excel Macro-Enabled Worksheets”, with a file extension of 

“.xlsm”. 

When the Excel template is first opened, users may get a warning at the top of the file, 

similar to the warning shown in yellow in Figure 53. Users should select the “Enable Content” 

button to ensure that GEM will run properly from this macro. 

Figure 53: Warning in Macro-Based GEM Files; Click the “Enable Content” Button to Run the Model 

Figure 54 shows the layout of the macro-enabled example input file for tractors. This 

input file layout in the first tab (labeled “GEM Data”) is identical to the layout of the tractor 

input file discussed in previous sections, except that this file includes “Validate” and “RUN 

GEM” buttons in cells A4 and B4. This file also includes two additional tabs that provide 

additional input for the user. The “Naming Structure” tab provides the naming convention for 

the regulatory subcategories. These names are the same as those seen in Table 17, Table 20, and 

Table 23 for the tractor, vocational and trailer categories, respectively. The “Input Limits” tab 

describes the input value ranges associated with each of the input fields. 

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Figure 54: Sample Microsoft Excel Macro Input File 

Users have two options to check for errors prior to running the model. The Excel macro 

is designed to catch some errors by displaying the erroneous fields with text that is red and 

crossed-out. If the user enters an input value that is out of range, uses lowercase letter, or repeats 

a unique identifier, the value will be replaced with red text with a line through it (see Figure 55). 

Figure 55: Sample Microsoft Excel Input File with Errors 

Users can also click on the “Validate” button in cell A4 to check for errors. Once the 

validation process is complete, the “_result.csv” and “_errors.txt” files will automatically be 

displayed. If a user does not fix errors identified in either of these methods, the model will still 

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run, but will produce errors in the output file and no certification results will be available for 

those runs. The macro will not provide a warning for mistyped supplemental file names (i.e., 

engine, transmission, or powertrain files) or incorrect file paths. Instead, it will produce a failed 

run for that simulation and the error message will note that it could not locate the file. Also, the 

model will not produce an error if a unique identifier is repeated, as seen in Figure 55 for the 

unique identifier “TRAC_PT2”. 

Once the inputs are properly specified in Excel, the user clicks the “RUN GEM” button 

to initiate the program. Please be patient. It may take a few seconds to begin. Users cannot 

make changes to the input file and may have limited access to Excel until the simulations are 

complete. Users can stop the simulation at any point by clicking the red “X” in the upper right 

corner of the status window. Users will be presented with a window to confirm that they want to 

end the simulation. The results and error logs will still be generated for the simulations that 

completed before the user chose to stop the model. 

Figure 56: Status Window to Show Progress of GEM Simulations 

Once complete, the result and error files will automatically be opened. When using the 

GEM Excel Macro, the output files will be renamed to match the date and time of the simulation, 

in the format: GEM_YYYYMMDD_HHmmSS_result.csv. The error file is saved with the 

same timestamp and a “_errors.txt” extension. 

Final Notes 

For more information on the rule, please see Docket EPA-HQ-OAR-2014-0827 available 

at http://www.regulations.gov. 

1

“Peer Review of the Greenhouse gas Emissions Model (GEM) and EPA's Response to Comments,” Docket # EPA420-R-15-009, June 2015. 

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