A sensor and method for measuring the volatility of liquid gasoline by estimating its driveability index includes a sensing element having an interdigitated array of electrically conducting capacitor plates arranged to retain a predetermined volume of gasoline, the volatility of which is to be measured. The sensing element is mounted in a vehicle to be in contact with the flow of gasoline while the engine is running so that a volume certain of gasoline is drawn between and remains within the electrically conducting plates when the engine is turned off. The sensing element is connected to circuitry used to measure the change in capacitance of the sensing element as a function of time while simultaneously measuring the temperature change of the sensing element as the volume of gasoline retained by the sensing element is evaporated over time. The measurements obtained by the circuitry are used in estimating the drivability index of the gasoline.