i intend to use a photo-interupter
Placed on what. Opto devices aren't normally very good for this sort of app because of the dirt under a vehicle.
read the rpm
RPM normally refers to the engine, I assume you are talking about a wheel or tail shaft.
the code itself is 29,000 chars
That's huge, I did a speedo with 6 trip meters in 7.5k (not Arduino though, raw AVR).
how i would run the code
Just plug it in and start making changes to match your application. If you copy the hardware there shouldn't be that ,many changes to make.
i would like to generate a display accurate to 0.1Km/h
I'd get something working first then look into that.