depending of your car model and year but most of the time, they already have a sensor called VSS (Vehicule Speed Sensor). It's actually pretty simple to read from them. Some of them will produce 6000 pulse/miles, some other 4000 and the list is long.

If your car have this Sensor, you can find the info at the link bellow
http://www.rostra.com/rostra-obstacle.htm --> Search application guide