Warm-up fuel consumption behaviour as affected by ambient temperature was evaluated for five OEM gasoline fuelled automobiles. Multiple EPA FTP 75 tests were performed with each vehicle at ambient test cell soak temperatures of 25°C and -7°C. Fuel consumption measured during the warm-up (Bag 1, Cold Transient) test segments at these two temperature conditions was compared to the fully warmed Hot Transient (Bag 3) fuel consumption from the 25°C ambient temperature tests (the Bag 1 and Bag 3 segments involve identical speed curves). Fuel consumption increases over the 25°C Bag 3 tests for the two warm-up test conditions were differentiated as those caused by increased drivetrain losses and those caused by intake charge enrichment.Results show wide variations in warm-up behaviour among the five vehicles with respect to the relative increases in fuel consumption, and the proportion of the fuel consumption increases attributable to drivetrain losses and enrichment. It was discovered that the most sophisticated vehicle systems do not necessarily facilitate the least degradation in fuel consumption with respect to baseline conditions for the group of vehicles tested.