Hi..

By what I understan of your case, you are trying to
measure the sparks delivered to the Plugs, this
result can give you the Round number of the Motor,
NOT THE WHEELS.

There is another, very simple way to measure the
MOTOR RPM its less accurate, but once you calibrate it
it'll be good for non electronics purposes...

Use CMOS INVERTER, connect its supply to 12V, take the
output from your ALTERNATOR (ELECTRICITY GENERATOR),
which generate AC Signal (usually around 15v).
and Supprise... its output frequency is linear to the
rounds of the motor (which connect directly by belt)
you can use 8051 or F2V to process the information.

Goodluck.
OK