In designing a regulatory vehicle simulation program for determining greenhouse gas (GHG) emissions and fuel consumption, it is necessary to estimate the performance of technologies, verify compliance with the regulatory standards, and estimate the overall benefits of the program. The agencies (EPA/NHTSA) developed the Greenhouse Gas Emissions Model (GEM) to serve these purposes. GEM is currently being used to certify the fuel consumption and CO2 emissions of the Phase 1 rulemaking for all heavy-duty vehicles in the United States except pickups and vans, which require a chassis dynamometer test for certification. While the version of the GEM used in Phase 1 contains most of the technical and mathematical features needed to run a vehicle simulation, the model lacks sophistication. For example, Phase 1 GEM only models manual transmissions and it does not include engine torque interruption during gear shifting. The engine control is simplified and does not include fuel cut-off during decelerations and the agencies pre-specified the engine fuel maps. These simplifications are acceptable as far as certification is concerned, since the Phase 1 certification process only requires up to five user inputs (aerodynamic drag coefficient, tire rolling resistance, vehicle speed limit, weight reduction, and idle reduction), and all other vehicle parameters and their inputs are pre-specified by the agencies.Moving forward, the proposed Phase 2 standards are predicated on the performance of a broader range of technological improvements than in Phase 1. Consequently, a more comprehensive vehicle simulation model is required. This paper describes a new version of this vehicle simulation model defined as Phase 2 GEM. In order to qualify this model, comprehensive validations were carried out against 130 vehicle variants including a number of medium- and heavy-duty trucks. Good agreement between simulation and test results were obtained, demonstrating that this model is technically sound and ready for the next phase of the GHG emission certification.