This calculator requires the use of Javascript enabled and capable browsers. This calculator is designed to determine the difference between the indicated speedometer speed and the actual speed after a change of gear ratio on the ring gear and pinion. This assumes no other changes such as tire or wheel size, engine changes or transmission changes. Enter the ratio of the original ring gear and pinion (changed from) and the new ring gear and pinion (changed to). Enter the indicated speed as read on the speedometer. Click on the Calculate button. At any given speedometer indicated speed, the true speed is the ratio difference multiplied by the speedometer indicated speed. Depending on the vehicle manufacturer and the transmission type and manufacturer, there are speedometer gears available to correct the indicated speed. Check with your manufacturer's parts supplier.