[Kevan]

There's a great free Android app called - Giri -The RPM Checker, remove the rear axle with gear and it'll fairly accurately tell you what RPM the motor is running by the noise, I've double checked it with a laser tacho and it's pretty close so the tacho doesn't get used now.

If you try to measure the RPM with the rear axle still in the car the extra noise will confuse the app as to what speed it is you're trying to measure, motor or rear axle.

...works best with the motor in your hand rather than in the car

Apparently also available for ifones