I would forget the PLC. Way overkill for what you want to do.
No sense in re-inventing the wheel, go to a junkyard and get the sensor and sensor gear from a wrecked car with ABS (anti-lock brakes). It's already setup for 12 volts, and should work quite well (and cheap). Connect it to a digital tach and you are home free.
BTW, I would look into large, skinny, high pressure wheels. Bicycle racing wheels for example. Mind how you turn though... (Hmmm, why not just use a bicycle speedometer from a bike shop?)
As an oldtimer wannabe, I personally would do the card-in-the-spokes trick (don't forget your Davey Crockett coonskin cap) and just listen to the pulses. But that's because I'm cheap.
As for cutting off the engine, I would humbly suggest that you could eyeball the speed and do this for yourself...
And evilmike, I mean elevmike, is right. It's better to keep the throttle in one place (arguably the futher from the floor the better), that's because most carburetors have an acceleration pump that squirts an extra shot of gas when the gas pedal is pressed.
Very wasteful.
Good luck
John