You can implement your speedometer easily with the PULSIN command.

You don't have to wait for a full revolution of you wheel.

Just install either magnets or reflective tape at equidistant intervals around your wheel and measure the time between any two magnets (or tape), keeping in mind that this measurement is only a fraction of a wheel revolution (and a fraction of a second). Handle the extrapolation in integer math.

I'm doing something similar with rotor tachometers and many other things on a helicopter. The software is written in a loop structure, such that all the measurements are taken and processed many times a second.