An official website of the United States government.

We've made some changes to EPA.gov. If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Regulations for Emissions from Vehicles and Engines

Greenhouse Gas Emissions Model (GEM) for Medium- and Heavy-Duty Vehicle Compliance


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.

You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.


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.

Top of Page


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, 693K, EPA-420-B-11-019, August 2011)
  • Download the executable version of GEM Setup (ZIP)(1 pg, 199 MB, July 2011)
    • GEM v2.0.1 (ZIP)(1 pg, 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, 1.67MB, EPA-420-R-11-007, August 2011) 
    This document contains the Peer Review Report, which is comprised of comments from four separate peer reviewers, and EPA’s response to those comments. 

Top of Page

Regulations for Emissions from Vehicles and Engines Home