Instrument Panel Cluster (IPC) Description and Operation: Speedometer
WARNING: This page is about a different car, the 2006 Buick LaCrosse and 2006 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
The instrument panel cluster (IPC) displays the vehicle speed based on Class 2 message from the powertrain control module (PCM)/engine control module (ECM). The speedometer defaults to 0 mph (0 km/h) or the last known state if the IPC detects a loss of class 2 communication with the PCM/ECM.