Body Control System Description and Operation: Odometer
WARNING: This page is about a different car, the 2004 Saturn L300. However, it is still accessible from the selected car via links, so may be relevant.
The BCM calculates vehicle mileage from rolling count data sent to the BCM by a CAN message from the ECM (L81) or a class 2 message from the PCM (L61). The BCM sends the mileage data to the cluster over serial data to display the vehicle mileage. The actual mileage is located in the BCM. All calculations for odometer are performed in the BCM.