You can use output from Hall sensor. Every modern car has Hall sensor, and older ones inductive sensor. They triger high voltage coil.

So, you get from Hall sensor max 12 V signal, with frequency: f = rpm * cyl * 0.5;

cyl - number of cylinders. You should have the same for high voltage, the difference is, that you should not care about signal conditioning, exept resistor devider ;)

Have fun ;)

Donatas