After installing tires that are larger or smaller than original equipment tires, or installing a ring and pinion gear set with a different rear gear ratio into your Dodge Ram 5.9L Cummins , the Speedometer Calibrator allows re-programming for correct speedometer and odometer readings, as well as correcting part-throttle shifting. As a result, the automatic transmission shift correctly and the speedometer readings will be accurate (as required by law).