Of all existing modes of transportation, onroad motor vehicles are the largest contributor to greenhouse gas emissions and fuel usage. The Environmental Protection Agency and the National Highway Traffic Safety Administration finalized regulations in April 2010 to reduce greenhouse gas emissions and improve fuel economy for 2012-2016 model year light-duty vehicles. In November 2010, both agencies jointly proposed the first ever greenhouse gas standards for medium- and heavy-duty trucks which are expected to take effect for model years starting in 2014.Vehicles of light-duty families are subject to mandatory testing for certification and compliance. Unlike the light-duty sector where a vast majority of vehicles are mass produced for generally similar purposes, medium- and heavy-duty vehicles are commonly custom-made. Permutations of engines, transmissions, axles, chassis frames, auxiliary equipment, and numerous other specific consumer requirements have resulted in tens or hundreds of thousands of truck configurations for some truck classes or applications in the fleet for any given year. To help manage the regulatory testing burden on truck manufacturers and the government agencies, the Environmental Protection Agency has created the Greenhouse gas Emissions Model which is to be jointly used by both agencies as the primary tool to certify vocational and combination tractor heavy-duty vehicles (Class 2b through Class 8 heavy-duty, excluding heavy-duty pickups or vans).This paper describes the simulation tool, including essential features and assumptions used in tool development. The model has been validated using Class 7 and Class 8 combination tractor data obtained from chassis dynamometer testing and benchmarked against a widely available commercial vehicle simulation tool.