[jimmcslim]

According to the Wikipedia page on speedometers, under the Error heading...

'Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits.', although no citation is currently provided.

Error is introduced by tire diameter being different than what was assumed for initial calibration... i.e. wear on the tires or tires that are under/over-inflated.

http://en.wikipedia.org/wiki/Speedometer