An improved engine control in which the fuel volatility is detected based on a measure of the fired-to-motored cylinder pressure ratio, and used to trim fuel and spark timing controls during engine warm-up and transient fueling periods. In a first embodiment, a matrix of empirically determined pressure ratio values that occur with fuels of differing volatility is stored and compared to the measured pressure ratio to identify the closest stored pressure ratio, and the fuel volatility is determined based on the fuel volatility associated with the identified pressure ratio. In a second embodiment, the actual fuel vapor-to-air equivalence ratio is computed based on the measured pressure ratio, and the fuel volatility is determined based on the deviation between the actual ratio and the desired fuel vapor-to-air equivalence ratio.