Wouldnt you want to find vehicle speed instead of engine rpm?
You can use either - assuming the tyres don't skid and there is no clutch slip there is a linear relationship between engine revs and speed on a car with a manual gearbox. It is different with an auto box.
You should monitor engine revs anyway and stop controlling when engine revs either change unexpectedly (clutch pedal depressed) or changes with no corresponding change in speed (skid, accident, flying over bridge etc).
This is an interesting project - one I have thought of doing many times. IMO, the hardware is the trickiest bit, so I would take the advice of seeing if you can get some hardware from a junk yard.