• Overview of GEM
• Phase 2 GEM simulation model
• Phase 1 GEM simulation model v2.0.1
• Proposed revisions to Phase 2 GEM simulation model

Overview of GEM

GEM is a free, desktop computer application that estimates the greenhouse gas (GHG) emissions and fuel efficiency performance of specific aspects of heavy-duty (HD) vehicles. GEM is designed to operate on a single computer.

Phase 2 GEM Simulation Model

(Supporting the Final HD Vehicle GHG Emissions and Fuel Efficiency Rules)

The model documentation provides details on how to install and use the model. The PDF file also contains the input files that were used to determine the stringency of the final GHG Emissions Standards and Fuel Efficiency Standards for Medium-duty and HD Vehicles. The downloadable installation file below contains the application executable files for the active version of Phase 2 GEM for simulating vehicle compliance.

• GEM User Guide for Phase 2: Vehicle Simulation Tool for Compliance with GHG Emissions Standards and Fuel Efficiency Standards for MD and HD Engines and Vehicles (PDF) (52 pp, 1.0 MB, July 2016, EPA-420-B-16-067, About PDF)
• Download the executable version of GEM_P2V3.0_setup_x64 (EXE) (604 MB, July 2016)
• GEM 3.5.1 User Guide for Phase 2: Vehicle Simulation Tool for Compliance with GHG Emissions Standards and Fuel Efficiency Standards for MD and HD Engines and Vehicles (PDF) (46 pp, 1.6 MB, December 2020, EPA-420-B-20-055, About PDF)
• Download the executable version of GEM P2v3.5.1-setup-x64.exe (EXE) (605 MB, December 2020)

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Phase 1 GEM Simulation Model v2.0.1

(Supporting the Final HD Vehicle GHG Emissions and Fuel Efficiency Rules)

The model documentation provides details on how to install and use the model. The PDF file also contains the input files that were used to determine the stringency of the final GHG Emissions Standards and Fuel Efficiency Standards for Medium-duty and HD Vehicles. The downloadable installation file below contains the application executable files for the active version of the GEM for simulating vehicle compliance. Lastly, the report of the peer review of GEM v1.0 includes the comments received on the model and EPA’s responses.

• Read the user guide for model architecture, how to use the model, and inputs used to determine the final standards (PDF) (16 pp, 693 K, EPA-420-B-11-019, August 2011, About PDF)
• Download the executable version of GEM Setup (ZIP)(1 pg, 199 MB, July 2011) Free Viewers
  • GEM v2.0.1 (ZIP)(199 MB, May 2013)
    A revision has been made to GEM to accommodate the revision to 49 CFR 535.6(b)(3) which derives the fuel consumption FEL from the CO2 FEL value rounded to the nearest whole number. The update also includes the previous update to address an installation bug has been identified for computers using a Windows 7 operating system. Due to the security setup of the Windows 7 operating system, the GEM executable code will not run from the default installation folder of C:\Program Files(x86)\EPA\GEM or from any other folder under C:/Program Files. This bug can be avoided by clicking on Browse under "Choose Installation Location," and then selecting a different local folder into which to install the GEM.
  • Please unzip the file first and then follow the GEM User Guide instruction to install GEM v2.0.1
• Peer Review Report and Response to Comments (PDF) (106 pp, 1.67 MB, EPA-420-R-11-007, August 2011, About PDF)
  This document contains the Peer Review Report, which is comprised of comments from four separate peer reviewers, and EPA’s response to those comments.

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Proposed Revisions to Phase 2 GEM Simulation Model

In December 2020, EPA proposed further revisions to the Phase 2 GEM 3.5.1 Simulation Model in the December 2020 Notice of Proposed Rulemaking (NRPM) for technical amendments to the GHG Emissions Standards and Fuel Efficiency Standards for Medium- and Heavy-Duty Vehicles and is soliciting comments on these revisions. EPA is releasing the following version of the model (GEM_P2v3.8) which incorporates the revisions being considered. This version is considered official only for use with the hardware in loop (HIL) test procedure defined in 40 CFR 1037.550, but is otherwise not an official version of GEM and it cannot be used for compliance with EPA emission standards.

• Supplemental Notice of Proposed Rulemaking for Improvements for Heavy-Duty Engine and Vehicle Test Procedures
• GEM 3.8 User Guide for Phase 2: Vehicle Simulation Tool for Compliance with the Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium and Heavy-Duty Engines and Vehicles – Phase 2 Technical Amendments Notice of Proposed Rulemaking (PDF) (56 pp, 1.9 MB, December 2020, EPA-420-B-20-056, About PDF)
• Download the executable version of GEM P2v3.8-setup-x64.exe (EXE) (607 MB, December 2020)

Additional Resources

• Notice of Proposed Rulemaking for Improvements for Heavy-Duty Engine and Vehicle Test Procedures, and other Amendments
• Final Rule for Improvements for Heavy-Duty Engine and Vehicle Test Procedures, and other Amendments
• Memorandum regarding additional unofficial version of GEM (V3.7) and potential adjustment factors (PDF)(3 pp, 75 K, October 2020, About PDF)

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