Body Control System Description and Operation: Odometer
WARNING: This page is about a different car, the 2003 Saturn Vue. 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. The BCM sends the mileage data to the cluster over class 2 serial data to display the vehicle mileage. The actual mileage is located in the BCM. All calculations for odometer are performed in the BCM.