When installing tires that are larger or smaller than original equipment tires, and/or installing a ring and pinion gear set with a different rear gear ratio, the Speedometer Calibrator allows you to correct speedometer readings. The vehicle's computer calculates your vehicle's speed based on the diameter of the stock tires. If you've installed taller or shorter tires, or changed the rear gear ratio, the vehicle's computer doesn't know how fast your vehicle is going. As a result, the speedometer readings will not be accurate (as required by law). Using the USB cable connect the In-line Speedometer Calibrator Module to your PC and enter the new tire height and/or rear gear ratio. Next, disconnect the factory connector to the instrument cluster (behind the dash), snap in the Speedometer Calibrator Module, reconnect the cluster, and you're done! In a matter of minutes, you can accurately correct your speedometer readings.