Taking your question at face value: depends on the sensor you want to use, and what interfaces it has. Maybe your on board computer has this data available.
Though if your real question is "what sensor is the simplest and fastest to work with" then it would be: attach magnet on the shaft, hall sensor above it, and start counting pulses. Should be one of the most direct ways. But if it works for YOUR car that depends on what kind of access you have to the motor and so.