This SAE Recommended Practice describes a procedure for measuring evaporative emissions from fuel systems of passenger cars and light trucks. Emissions are measured during a sequence of laboratory tests that simulate typical vehicle usage in a metropolitan area during summer months: 1 A 1 h soak representing one diurnal cycle in which temperature of fuel in the vehicle’s tank is raised from 60 to 84 F (15.6 to 28.9 C). 2 A 7.5 mile (12.1 km) run on a chassis dynamometer. 3 A 1 h hot soak immediately following the 7.5 mile (12.1 km) run. The method described in this recommended practice for measuring the weight of fuel vapors emitted during the tests differs from that described in SAE J170. SAE J170 employs activated carbon traps connected to the fuel system at locations where vapors are expected to escape. Vapors from these openings are absorbed by the traps, and the gain in weight of the traps represents the fuel evaporative emissions.