you mount a tiny magnet to your flywheel or gear to get the reading off of.

you have to mount a sensor in close proximity to the magnet, then you measure the distance the car rolls in one revolution of the tires, input it into the speedometer and you are good to go. The hardest thing is the mount for the sensor, I ended up using the plastic box that bullets come in, I cut it to fit with my dremel and it works great..