Don't mean to give you a lesson in automotive mechanics and engineering, but since you asked for it....If you want to measure the speed of a rotating shaft with an accelerometer you will have to mount the accelerometer on the rotating component. How will you then take the signal off of it? Yes, you can get a wireless accelerometer, but that would be silly. The speed of a rotational shaft will best be measured with an optical or magnetic pickup that counts revolutions (revolutions/time is proportional to velocity). The neat thing about this method is that the sensor is mounted in a fixed position, and wiring isn't complicated by the rotating shaft.

This isn't about physics, or design. It is about automotive diagnosis and repair. The best and most suitable diagnostic aid you have available is a tire pressure gauge.

Cheers

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue