The National Vehicle and Fuel Emissions Laboratory (NVFEL) tests a portion of all new cars and trucks to confirm compliance with EPA’s exhaust emissions standards, and also to verify the accuracy of fuel economy test results submitted by each manufacturer. NVFEL also conducts testing of used vehicles, usually between one and seven years old, to survey the effectiveness of the vehicle’s emissions control systems over 120,000- to 150,000-mile useful life. Vehicles are tested on a dynamometer under standard test cycles designed to account for various driving conditions, including city, highway, and aggressive driving, as well as cold and hot weather conditions. 

More information on regulations covering vehicle testing procedures.

More information on dynamometer vehicle driving cycles: Dynamometer Drive Schedules.