If you're running non-stock tire sizes and/or gear ratios. Hypertech's Speedometer Calibrator can correct the speedometer and odometer for tires up to 54 inches (depending upon application) and non-stock rear gear ratios. It plugs into the vehicle's under dash diagnostic port, and in a matter of minutes, the vehicle's computer is reprogrammed for accurate readings. The Speedometer Calibrator does NOT contain performance tuning or other performance options.