Information on Speedometer Accuracy

This bulletin is being revised to add the GMC Acadia. Please discard Corporate Bulletin Number 08-08-49-011A (Section 08 - Body and Accessories).

The speedometer used on most GM vehicles are of the Air-Core design. This is a moving needle operated by a magnetic coil. Because there is no direct mechanical linkage, there is some variation in accuracy that cannot be eliminated.

Digital readout devices, such as some speedometer displays and Head-Up Displays (HUD), are direct readouts of the Vehicle Speed Sensor (VSS) and their accuracy is much greater. Tire size, tread life and inflation may cause some variation in the readings. In addition, the Tech 2® reads the VSS directly and is quite accurate.

Questions on speedometer accuracy can usually be resolved by comparing the speedometer reading to the Tech 2 while driving. The following table lists allowable speedometer variations. Any Air-Core speedometer that reads within these limits should not be returned for service.