Variation of gasoline's driveability index (DI) limits control of the air-to-fuel ratio during cold starts. The DI of fuel purchased at the pump is correlated with ambient temperature. The DI variability that remains after accounting for this correlation is quantified for fuel samples collected in 1998. A new type of sensor to measure DI is introduced. The sensor is located in the fuel tank, above the highest liquid level. A small sample of fuel is heated at the end of each trip. Capacitance measurement is used to determine oxygenate concentration and to monitor the evaporation of the sample as a function of temperature. The sensor has been used to determine the DI of fuel on-board a vehicle.