Diesel Vehicle Classes
Motor vehicles are separated into different classes for various statistical and regulatory purposes, typically based on gross vehicle weight rating (GVWR). GVWR is the maximum allowable total weight of a loaded vehicle. These classifications are important because they affect the availability of diesel vehicles in the United States and the standards those vehicles must meet.
Light-Duty Vehicles 
The U.S. Environmental Protection Agency distinguishes between light- and medium/heavy-duty vehicles for the purposes of emissions regulations. All vehicles of less than 8,500 lb GVWR are considered light-duty vehicles and are subject to light-duty vehicle emissions regulations (in grams of emissions per mile). Passenger vehicles of 8,500 to 10,000 lb GVWR—primarily large sport utility vehicles and passenger vans—are also subject to light-duty vehicle emissions regulations.
Light-duty vehicles are also subject to Corporate Average Fuel Economy (CAFE) standards (in miles per gallon, mpg). These standards require each car manufacturer to achieve a specified sales-weighted fuel economy for its entire fleet of vehicles sold in the United States in each model year. In the future, CAFE standards will apply to all passenger and non-passenger vehicles up to 10,000-lb GVWR manufactured for U.S. sale.
For detailed information, see DieselNet's Emission Standards and CAFE pages.
Medium- and Heavy-Duty Vehicles
Engines for medium- and heavy-duty vehicles undergo separate emissions testing and certification (in grams of emissions per brake horsepower-hr, g/bhp-hr). Commercial vehicles of 8,500 to 10,000 lb GVWR are subject to medium-duty engine emission regulations.
For other regulatory and statistical purposes, vehicles are divided into eight classes based on GVWR. Medium-duty and "light heavy-duty" vehicles typically are defined as classes 3 to 6 (10,001 to 26,000 lb GVWR) and heavy-duty vehicles as classes 7 to 8 (greater than 26,000 lb GVWR).