Originally Posted by dedwards0323

The speed sensors for the XFire are located in the Transmission Solenoid Assembly (TSA) which is mounted & connected to the Transmission Control Module (TCM). There are 2 sensors in the transmission (NAG1 Auto-Shift). Since these serve as the primary inputs to the overal speed control system, the drive-train plays a major part in the overall electronic speed control of the vehicle, which is a "no brainer". (Note: The cruise control system gets inputs from additional multiple sources to control the vehicle speed.) So by changing the rear wheel/tire geometry, one does imact the accuracy of the speedometer readout. But the change is so minor over the range of tire sizes available, most folks wouldn't see a difference (see attached).

What I wasn't able to identify is whether the electronics are sophisticated enough in our XFires for the ECM to automatically adjust the Speedo readout for a detectable speed difference seen by the tranny speed sensors when a different rear wheel/tire setup has been installed. In theory, if any difference was seen by the ECM compared to factory setup, the ECM could make an adjustment so the Speedo maintained its factory accuracy.