Well, the manual speedometer will be hooked up mechanically by cable. This is usually done at the Transfer case, through a speedometer gear.
For an electric speedometer, you still have a speedometer gear, but instead of a mechanical cable you have the Vehicle Speed Sensor (a hall effect sensor), to determine the vehicle speed and report it to the PCM (Powertrain Control Module) and to the electronic speedometer head.
Why go backwards from electric to mechanical? The PCM most likely depends on the signal from the Vehicle speed sensor (VSS) to control other functions than just the speedometer (for example: ABS system). So, you'd probably Not want to elimanate the VSS.
If you are concerned about the speedometer's accuracy, changing the speedometer gear is the solution for recalibrating either an electric or mechanical head.
-Nick :!:
