Depending on your model and setup. I'm assuming 4x4. Your speed signal will be coming from a hall effect sensor either on the transmission or the rear end housing. This signal is a pulse signal. The GEM (2wd pre-2001) or 4WABS (4x4 or 2001-up 2wd) will convert this pulse signal into a voltage that is sent to the cluster. This voltages splits to the speedometer and the odometer so they both read the same. I've noticed that factory readings are always off about 2 units between odometer and speedometer. Odometer usually reads dead on but the speedometer always reads about 2 mph faster than what you are really going.
If your a real accuracy stickler I would get your odometer to read correctly and then adjust the needle on the speedometer manually (removing and setting cruise to a known speed and re applying the needle).
Note GPS speedometers are acurate to about 0.1 of a mile.
Quality Engineer (Electronics) for Kia Motors
University of Tennessee Electrical Engineering Graduate
2000 XLT 2.5L A/T