This is a good way to determine engine RPM without mechanical modifications to the car. Just make sure you modify the code for the correct number of cylinders in your vehicle.

The parts arrived for the RPM meter that uses an Arduino and reciprocal counting to get a three-times-a-second update rate with 1rpm resolution. Arduinos ...

The DEC 24/1 brushless dc (BLDC) motor handles continuous current up to 0.95 mNm and delivers a nominal speed up to 80,000 rpm with an efficiency of 70%.