Greenhouse Gas Emissions Model 

(GEM) User Guide 
Greenhouse Gas Emissions Model 

(GEM) User Guide 

Assessment and Standards Division 

Office of Transportation and Air Quality 

U.S. Environmental Protection Agency 

EPA-420-B-11-019 

August 2011 
 

 

 

 

 

 

 

 

The Greenhouse gas Emissions Model was created by EPA 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. The GEM v2.0 model 

itself is a part of the final regulation.

1 It is a free, desktop computer application. 

The GEM is designed to operate on a single computer. The downloadable installation 

file contains the application executable file and other supporting files, available at 

http://www.epa.gov/otaq/climate/gem.htm, and described in this guide. 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. 

After the GEM version 1.0 was released to the public in October 2010 as a part of the 

proposed rules, the agencies received many constructive comments and useful suggestions from 

the public. Prior to promulgation of the final rules, the agencies continued validations against 

additional test data and benchmarking against other commonly used industrial standard vehicle 

models. The most noticeable improvements to the GEM version 2.0 are a new driver model, the 

simplified electric model in conjunction, and many other refinements including the graphical 

user interface. 

Chapter 4 of the RIA details model validations against testing data and comprehensive model 

benchmark against commonly used vehicle model in industry. This user guide contains the 

model documentation with details on the model’s input and output files. It also includes 

instructions on how to use the GEM. Some of the information provided here can also be found 

in Chapter 4 of the RIA. 

1. GEM Documentation 

This section describes the GEM vehicle model architecture, the list of pre-defined input 

parameters, output calculations, and the installation of executable GEM code into the user’s 

computer. 

1.1. Vehicle Model Architecture 

The Class 2b-8 vehicle compliance model architecture is comprised of six systems: 

Ambient, Driver, Electric, Engine, Transmission, and Vehicle. With the exception of 

“Ambient,” “Electric,” and “Driver,” each system consists of one more component models. 

Details on the function of each system and their respective component models can be found in 

Chapter 4 of the RIA. 

1 See Sections II.B and II.D of the preamble and Chapter 4 of the RIA (Regulatory Impact Analysis), available at 

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

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1.2. List of Predefined Input Parameters for Class 7/8 Combination 

Tractor Models 

Although many technologies can potentially achieve GHG emission and fuel 

consumption reductions, EPA and NHTSA believe that for the rule’s timeframe, some may be 

too complex to model for compliance purposes (e.g., hybrid control) while others require 

standardization such as the calculation of GHG and fuel consumption benefits due to 

aerodynamic improvements. EPA and NHTSA decided to pre-define the tractor frontal area, 

tractor-trailer combined and payload weight, gear box and its efficiency, final drive ratio, 

engine/transmission/wheel inertia, accessory load, axle base, tire radius, trailer tire coefficient of 

rolling resistance (Crr, trailer tires), and engine fuel map. The agencies use these pre-defined 

input parameters in the simulation model for all subcategories of combination tractors. Table 1 

lists the specific values of these parameters for Class 8 combination tractors, and Table 2 lists the 

similar values for Class 7 combination tractors. 

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Table 1: Class 8 Combination Tractor Modeling Parameters 

MODEL TYPE CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 8 

Regulatory 

Subcategory 

Sleeper Cab 

High Roof 

Sleeper Cab 

Mid Roof 

Sleeper Cab 

Low Roof 

Day Cab 

High Roof 

Day Cab 

Mid Roof 

Day Cab 

Low Roof 

Fuel Map 15L - 455 HP 

Gearbox 10-speed 

Manual 

10-speed 

Manual 

10-speed 

Manual 

10-speed 

Manual 

10-speed 

Manual 

10-speed 

Manual 

Gearbox Ratio 14.8, 10.95, 8.09, 5.97, 4.46, 3.32, 2.45, 1.81, 1.35, 1 

Gearbox 

Efficiency 0.96, 0.96, 0.96, 0.96, 0.98, 0.98, 0.98, 0.98, 0.98, 0.98 

Engine Inertia (kgm2

) 4.17 4.17 4.17 4.17 4.17 4.17 

Transmission 

Inertia (kg-m2) 5 5 5 5 5 5 

All Axle Inertia 

(kg-m2) 360 360 360 360 360 360 

Loaded Tire 

Radius (m) 0.489 0.489 0.489 0.489 0.489 0.489 

Tractor Tare 

Weight (lbs) 19,000 18,750 18,500 17,500 17,100 17,000 

Trailer Weight 

(lbs) 13,500 10,000 10,500 13,500 10,000 10,500 

Payload (lbs) 38,000 38,000 38,000 38,000 38,000 38,000 

Total weight (lbs) 70500 66750 67000 69000 65100 65500 

Total weight (kg) 31978 30277 30391 31298 29529 29710 

Frontal Area (m2

) 10.4 7.7 6.9 10.4 7.7 6.9 

Coefficient of 

Aerodynamic Drag OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input 

Axle Base 5 5 5 5 5 5 

Electrical 

Accessory Power 

(W) 

350 350 350 350 350 350 

Mechanical 

Accessory Power 

(W) 

1,000 1,000 1,000 1,000 1,000 1,000 

Final Drive Ratio 2.64 2.64 2.64 2.64 2.64 2.64 

Tire CRR 

(kg/metric ton) = 0.425 × Trailer CRR + 0.425 × Drive CRR + 0.15 × Steer CRR 

Trailer Tire CRR 

(kg/metric ton) 6 6 6 6 6 6 

Steer Tire CRR 

(kg/metric ton) OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input 

Drive Tire CRR 

(kg/metric ton) OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input 

Vehicle Speed 

Limiter (mph) OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input 

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Table 2: Class 7 Combination Tractor Modeling Parameters 

MODEL TYPE CLASS 7 CLASS 7 CLASS 7 

Regulatory Subcategory Day Cab High 

Roof 

Day Cab Mid 

Roof 

Day Cab 

Low Roof 

Fuel Map 11L - 350 HP 

Gearbox 10-speed 

Manual 

10-speed 

Manual 

10-speed 

Manual 

Gearbox Ratio 11.06, 8.19, 6.05, 4.46, 3.34, 2.48, 1.83, 1.36, 1, 

0.75 

Gearbox Efficiency 0.96, 0.96, 0.96, 0.96, 0.98, 0.98, 0.98, 0.98, 0.98, 

0.98 

Engine Inertia (kg-m2) 3.36 3.36 3.36 

Transmission Inertia 

(kg-m2) 5 5 5 

All Axle Inertia (kg-m2) 233.4 233.4 233.4 

Loaded Tire Radius (m) 0.489 0.489 0.489 

Tractor Tare Weight (lbs) 11,500 11,100 11,000 

Trailer Weight (lbs) 13,500 10,000 10,500 

Payload (lbs) 25,000 25,000 25,000 

Total weight (lbs) 50000 46100 46500 

Total weight (kg) 22680 20910 21092 

Frontal Area (m2

) 10.4 7.7 6.9 

Coefficient of Aerodynamic 

Drag OEM Input OEM Input OEM Input 

Axle Base 4 4 4 

Electrical Accessory Power 

(W) 350 350 350 

Mechanical Accessory 

Power (W) 1000 1000 1000 

Final Drive Ratio 3.73 3.73 3.73 

Tire CRR 

(kg/metric ton) 

= 0.425 × Trailer CRR + 0.425 × Drive CRR + 

0.15 × Steer CRR 

Trailer Tire CRR 

(kg/metric ton) 6 6 6 

Steer Tire CRR 

(kg/metric ton) OEM Input OEM Input OEM Input 

Drive Tire CRR 

(kg/metric ton) OEM Input OEM Input OEM Input 

Vehicle Speed Limiter 

(mph) OEM Input OEM Input OEM Input 

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1.3.	 List of Predefined Input Parameters for Class 2b-8 Vocational 

Vehicle Models 

Likewise, EPA and NHTSA standardized a set of parameters for the three Class 2b-8 

vocational vehicle types, which the agencies refer to as Vocational Light Heavy-Duty (VLHD), 

Vocational Medium Heavy-Duty (VMHD), and Vocational Heavy Heavy-Duty (VHHD). These 

predefined parameters include the coefficient of aerodynamic drag, truck frontal area, truck total 

and payload weight, the gear box and its efficiency, final drive ratio, engine/transmission/wheel 

inertia, accessory load, axle base, tire radius, and the engine fuel map. The specific values of 

these parameters are listed in Table 3. 

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Table 3: Vocational Vehicle Modeling Parameters 

Model Type Heavy Heavy-Duty Medium Heavy-Duty Light Heavy-Duty 

Regulatory Subcategory Vocational Truck 

(Class 8) 

Vocational Truck 

(Class 6-7) 

Vocational Truck 

(Class 2b-5) 

Fuel Map 15L - 455 HP 7L - 270 HP 7L - 200 HP 

Gearbox 10-speed Manual 6-speed Manual 6-speed Manual 

Gearbox Ratio 14.8, 10.95, 8.09, 5.97, 4.46, 

3.32, 2.45, 1.81, 1.35, 1 

9.01, 5.27, 3.22, 2.04, 

1.36, 1 

9.01, 5.27, 3.22, 2.04, 

1.36, 1 

Gearbox Efficiency 0.96, 0.96, 0.96, 0.96, 0.98, 

0.98, 0.98, 0.98, 0.98, 0.98 

0.92, 0.92, 0.93, 0.95, 

0.95, 0.95 

0.92, 0.92, 0.93, 0.95, 

0.95, 0.95 

Engine Inertia (kg-m2

) 4.17 2.79 2.79 

Transmission Inertia 

(kg-m2) 5 0.5 0.5 

All Axle Inertia (kg-m2

) 200 60 60 

Loaded Tire Radius (m) 0.489 0.389 0.378 

Payload (lbs) 15,000 11,200 5,700 

Total weight (lbs) 42,000 25,150 16,000 

Total weight (kg) 19,051 11,408 7,257 

Frontal Area (m2

) 9.8 9.0 9.0 

Coefficient of 

Aerodynamic Drag 0.7 0.6 0.6 

Axle Base 3 2 2 

Electrical Accessory 

Power (W) 350 300 300 

Mechanical Accessory 

Power (W) 1,000 1,000 1,000 

Final Drive Ratio 2.64 3.36 2.85 

Tire CRR 

(kg/ton) = 0.5 × Drive CRR + 0.5 × Steer CRR 

Trailer Tire CRR 

(kg/metric ton) Not applicable Not applicable Not applicable 

Steer Tire CRR (kg/metric 

ton) OEM Input OEM Input OEM Input 

Drive Tire CRR 

(kg/metric ton) OEM Input OEM Input OEM Input 

1.4. Output Processes 

The outputs produced by the GEM include post processes to calculate the final weighted 

results. 

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The GEM produces a cycle-weighted grams CO2/ton-mile and gallons/1,000 ton-mile 

result which incorporates the drive cycle weightings of the ARB transient cycle, 55 mph steady 

state cruise, and 65 mph steady state cruise cycle, as shown in Table 4. 

Table 4: Drive Cycle Weighting 

CATEGORY 

CLASS 8 

SLEEPER CAB 

TRACTORS 

CLASS 7/8 

DAY CAB 

TRACTORS 

CLASS 2b-8 

VOCATIONAL 

VEHICLES 

ARB Transient 5% 19% 42% 

55 mph Cruise 9% 17% 21% 

65 mph Cruise 86% 64% 37% 

The GEM converts the mile per gallon result into ton-mile space by using the payload for 

each regulatory class – 19 tons for Class 8 tractors, 12.5 tons for Class 7 tractors, 7.5 tons for 

HHD vocational vehicles, 5.6 tons for MHD vocational vehicles, and 2.85 tons for LHD 

vocational vehicles. 

GEM calculates the gallons/1,000 ton-mile weighted result by converting the weighted 

grams CO2/ton-mile result. The gram CO2/ton-mile result is multiplied by 1,000 and divided by 

10,180 grams CO2 per gallon of diesel fuel. 

2. GEM Installation Instructions 

Certain minimum computer requirements must be met before installation. The system 

required to run GEM needs a minimum RAM of 1 GB (4 GB is highly recommended), and 

approximately 900 MB of disk storage. Window XP or Window 7 operating system is required. 

The validation has not been tested in other computer operating systems. The 2007 Microsoft 

Excel or later version is required. 

The executable form of GEM named “GEM_Setup.exe” can be downloaded from the 

EPA website, at http://www.epa.gov/otaq/climate/gem.htm. Before installation, please close all 

other programs. Follow the procedure below to perform installation. 

•	 Down load GEM_Setup into a temporary directory. 

•	 Click GEM_Setup.exe to install the package and GEM Installation window will be 

displayed. 

•	 Click Next; a “Choose Install Location” window will appear. The user has the option to 

select the folder location or just use default folder on screen. Click Next. 

•	 A “Select Shortcuts” window will be displayed. A default checkbox for Create a 

Desktop Icon is already selected. Click Next. 

•	 A Window “Ready to Install” will be displayed. Click Install. 

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In the next few steps, it will install 2008 Microsoft Visual C++ Redistributable Setup and 

the MATLAB® Runtime 7.15 if you have not installed those previously. If these two were 

installed previously, click Cancel once Choose Setup Language window displays. The 

installation is finished. 

For Microsoft Visual C++ Redistributable Setup, just follow on-screen instruction to 

install. If Visual C++ Redistributable Setup has been installed previously, the installation will 

not prompt you to install, and you just go to the next step for MATLAB® Runtime 7.15 

installation. 

For MATLAB® Runtime 7.15 installation, follow the steps below. 

•	 Installation of MATLAB® Runtime 7.15 will be automatically initiated either right after 

installation of Visual C++ Redistributable Setup or click OK once Choose Setup 

Language window displays. 

•	 An InstallShield Wizard window will be displayed, prompting the user to install the 

MATLAB® Runtime 7.15. Click Next. 

•	 Click Install, and then follow the steps below to complete the installation 

a.	 Enter the names of user and organization, and then click Next. 

b.	 A window “Destination Folder” will be displayed. Click Next if a default folder is 

selected. 

c.	 Click Install to complete MATLAB® Compiler Runtime. It will take a while to 

complete the installation. 

d.	 Click Finish, and entire installation is completed. 

The GEM folder contains these visible files: 

•	 GEM.exe shortcut 

•	 GEM_input.xlsx 

It should be pointed out that the GEM is compatible with MATLAB® Runtime 7.15. 

Different versions of MATLAB® Runtime may result in compatibility issues. 

To uninstall, the user may manually delete the GEM by following the steps below: 

•	 Delete the GEM folder entirely 

•	 Delete the short cut on your desktop screen. 

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3. GEM Running Instructions 

To run GEM, double click the GEM.exe on the desktop icon. A graphic user interface 

GUI will pop as shown in Figure 1. 

Figure 1 – GEM Input GUI for Single Configuration Run 

With the GEM GUI launched, the user has two options to run the GEM. The first option 

is to run single configuration, and the second option is to allow the user to run multiple 

configurations or a batch job under the same regulatory subcategory and vehicle model year. 

For the single configuration, use the following instruction: 

Figure 1 allows the user to enter the required parameters into the model. There are 

essentially two types of input parameters. The first set of parameters is required to provide 

information to EPA and NHTSA. These include the following: 

• Manufacturer Name 

• Date 

• Vehicle Family 

• Vehicle Configuration 

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•	 Vehicle Model Year 

The second set of parameters affects how GEM calculates the final results. The list 

below describes each input and how it is used within GEM. 

•	 Vehicle Model Year: The pull-down allows the user to select five model years: “2013”, 

“2014”, “2015”, “2016”, and “2017 and later”. The Vehicle Model Year selects the 

appropriate engine fuel map in the model. The 2013 to 2016 model years all use one set of 

heavy-duty vehicle’s engine fuel maps which meets the 2014 through 2016 model year 

engine standards, while a selection of 2017 and later uses another set of the engine fuel maps 

which meet the 2017 MY engine standards. 

•	 Regulatory Subcategory: The user must select one of the twelve designated regulatory 

subcategories in the radio button menu. The selection directs the model to use the 

appropriate predefined inputs and post processing parameters, as outlined in Sections 1.2 and 

1.3 of this Guidance. 

•	 Coefficient of Drag: The Cd value is input by the user based on the aerodynamic bins, as 

discussed in the preamble, RIA, and regulations. This input is only required for combination 

tractors. No input is required for vocational vehicles. 

•	 Steer Tire Rolling Resistance: The coefficient of rolling resistance for the steer tires should 

be input by the user in the units of kg/metric ton where the typical value is greater than 5.5 

kg/metric ton. The units are not in kg/kg or lb/lb (the other industry norm) where the values 

are typically greater than 0.0055 kg/kg or lb/lb. 

•	 Drive Tire Rolling Resistance: The coefficient of rolling resistance for the drive tires (i.e. all 

tires used in non-steer positions) should be input by the user in the unit of kg/metric ton 

where the typical value is greater than 5.5 kg/metric ton. The units are not in kg/kg or lb/lb 

(the other industry norm) where the values are typically greater than 0.0055 kg/kg or lb/lb. 

•	 Vehicle Speed Limiter: If the vehicle contains a vehicle speed limiter, then input the speed 

limit to the nearest single decimal place. Otherwise, use 65 mile/hour. The GEM will limit 

the maximum speed of the vehicle to the value selected. If the user is opting to allow an 

expiration of the VSL before the full useful life of the vehicle or include soft top features, 

then calculate the Effective Speed Limit as described in 40 CFR 1037.640(d). This input is 

only available for combination tractors. No input is allowed for vocational vehicles as 

indicated by a “grayed-out” feature. 

•	 Vehicle Weight Reduction: If a combination tractor contains lighter weight wheels or tires, or 

other lighter components described in the preamble, RIA, and the regulations, then the user 

would input the sum of the weight reductions prescribed by the weight bins. This input is 

only available for combination tractors. No input is allowed for vocational vehicles as 

indicated by a “grayed-out” feature. 

•	 Extended Idle Reduction: If a sleeper cab combination tractor contains an extended idle 

reduction technology and a 5 minute automatic engine shut-off, then the user would select 

the 5 g/ton-mile reduction. If not, then “0” should be selected. If the user is opting to allow 

an expiration of the automatic engine shut-off before the full life of the vehicle, then 

calculate the Extended Idle Reduction Credit as described in 40 CFR 1037.660(c) This input 

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is only available for sleeper cab combination tractors. No input is allowed for day cab 

combination tractors and vocational vehicles as indicated by a “grayed-out” feature. 

•	 A check button for “Plot Output” can be selected if the user chooses to output all three cycle 

plots for vehicle speed traces as a function of time. 

• Click RUN (Figure 1), the GEM will run. 

After the simulation is completed, a window will open, prompting the user where the file 

is to be saved. The output file will open once clicking OK to save the file. The following output 

file will be created with the following naming structure: 

GEM_Results_Date_Time.xlsx 

The data are in the format of date-month-year, and Time is hour-minute-second-am/pm. 

For example, the full name is 

GEM_Results_01-Jun-2011_02-25-39-pm.xlsx 

The output file has the same structure as the input file, which includes all data used in the 

input file plus additional columns for output results. Those additional data are CO2 in g/ton-mile 

and fuel consumption in gallon/1,000-ton-mile, which will be used for certification. 

If the user wants to close the GEM, click on the GUI window. 

For single configuration run, input error checking features have been built in. If the user 

forgets entering a value to a required field, a message window will open and the field that 

requires entering the values will be in red, asking the user to enter a valid value. 

For multiple configurations, use the following instruction 

The user must set up the input file in Excel format before initiating “Multiple 

Configuration” run. A template input file is provided during installation. 

It should be pointed out that there are a few restrictions in using the EPA provided input 

template Excel file (GEM_Input.xlsx ) during editing before running the multiple configuration. 

•	 Only one regulatory subcategory and one vehicle model year can be selected for the batch 

process. 

• Do not change any format used in the template provided. 

•	 It is the user’s responsibility to enter the appropriate, valid numerical value for each 

mandatory input, which should not be left blank. 

•	 For all vocational vehicles selected in the “GEM_Input.xlsx”, those input parameters for 

“Cd (Bin)”, “Idle Reduction”, and “Weight Reduction” will take pre-defined default 

values irrespective of the values entered by the user. Those default values can be seen 

through “Single Configuration” run when the vocational trucks are selected in their 

respective grayed-out areas. 

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•	 For all day cab tractors including Class 7 and 8, the input parameter for “Idle Reduction” 

will use the pre-defined values irrespective of the values entered by the user. The predefined value can be seen through “Single Configuration” run when the vocational 

vehicles are selected in their respective grayed-out area.

 One example of the input file can be seen in Figure 2. Note that the values in this figure 

and other similar figures in the following section are for illustrative purposes only. The template 

provided by the agency is called “GEM_Input.xlsx”. Following the steps below, one can create 

the input file. 

Figure 2 Example of Input file for Multiple Configuration Run 

•	 Enter Manufacturer and Vehicle Family Names. 

•	 Select the Model Year in row 2 and column B through a pull-down menu. 

•	 Select one of twelve regulatory subcategories in row 4 and column B through a pulldown menu. 

•	 Enter an input file name. 

•	 Fill out the input parameters required by the GEM, such as Cd(Bin), CRR, Idle 

Reduction, Weight Reduction, and VSL. The instructions on how to enter these values is 

the same as those illustrated in the Single Configuration set up in the previous section. 

•	 It is the user’s responsibility to input “configuration”, “Brand/Model Name”, “Bin”, and 

“CdA (Test)” for each case. These values are not used by the GEM during simulation, 

and they are only used for reference during certification. 

•	 Save the file. 

•	 Go back to GEM GUI (Figure 1). 

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• Click “Multiple Configurations” in Figure 1. A window that says “Select the xlsx input 

file for Multiple Configurations” will display as follows (Figure 3). 

Figure 3 GEM Input File Selection 

• Select the folder where the input file is located. Click the file, and then click open. 

• The following window will appear. Click OK. The input file setup is completed. 

•	 The RUN button in Figure 1 will be changed to RUN BATCH. Click RUN BATCH. 

Multiple Configurations will then run. A progress bar window will be displayed. 

•	 Once simulation is completed, a window will open, prompting the user where the file is 

to be saved. Click OK to save the output file in your desired folder. 

•	 After a few seconds, a window indicating completion of simulation will appear as 

follows. Click Ok. In the mean time, the Excel output file will open. 

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The same output file structure as the single configuration run is used, but with more 

results depending on the total number of cases running. 

If the user wants to close the GEM program, click on the GUI window. 

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