Instrument Cluster Description and Operation: Speedometer
WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The IPC displays the vehicle speed as determined by the ECM. The IPC calculates the mileage based on the GMLAN vehicle speed information from the ECM. The speedometer defaults to 0 km/h (0 mph) if the IPC detects a loss of GMLAN communications with the ECM.