Many passenger cars exhibit various driveability malfunctions during cold start operation. These engine malfunctions, such as stalling, hesitation, stumble, and backfire, can usually be lessened or eliminated with increased ambient temperatures and increased gasoline volatility. Based upon laboratory performance testing of 99 cars, we have developed a Driveability Number (DNT), which is a gasoline quality parameter that can be used to predict the driveability performance of a fleet of cars. DNT is calculated from a model which is a function of ambient temperature and gasoline volatility. The gasoline volatility parameters used in this model are the 10%, 50%, and 90% evaporation points. The model was developed using late-model (1976-1982) cars, which were tested under controlled environmental conditions on a chassis dynamometer.