The highly preferred SAE Recommended Practice for measuring evaporative emissions from fuel systems of passenger cars and light trucks is the enclosure technique detailed in SAE J171. The sensitivity and accuracy of the enclosure technique is superior to that of the trap method. This recommended practice is retained for historical reference and for use with older vehicles imported into the United States of America. In addition, this trap method is referenced in SAE J171a for making running loss measurements which cannot practically be made in an enclosure. Emissions are measured during a sequence of laboratory tests that simulate typical vehicle usage in a metropolitan area during summer months.