A vehicle’s speedometer is in charge of giving an accurate estimation of the speed it is driving. When buying a car from a dealer, the speedometer is adjusted, or calibrated to the vehicle’s stock parts. If the owner decides to change the stock parts, such as the vehicle’s tires, it can throw of the speedometers calibration, giving the driver the wrong reading for the vehicle’s speed. Switching to larger tires causes the speedometer to give off a slower speed reading than the vehicle’s actual speed. Similarly, switching to smaller tires causes the speedometer to give a faster reading for the vehicle’s speed. After the tires are changed, the speedometer needs to be recalibrated. Here are a few simple steps on how to change speedometer calibration for a mechanical or electrical speedometer.
If you have a mechanical speedometer you would start by opening the vehicle’s hood and locating the transmission. Next, unscrew the speedometer cable and unscrew the bolt holding the gear housing cover. After removing the plate, you will see two gears. Count the number of teeth the gears have and remember it for later. Next, get the diameter of the tire and divide 20,168 by the tire’s diameter. This is the tire’s revolutions per mile. Multiply this by the amount of teeth your gears have and multiply again by the vehicle’s axle ratio (which can be found in the vehicles supporting documents). Purchase the necessary driven gear for your vehicle. Remove the old driven gear by pulling it out of its clips and clip in the new driven gear. Secure the gear housing cover and the speedometer cable back.
If you have an electric speedometer you would start by determining the distance of the test drive you would need to recalibrate your speedometer, which can be found in the vehicles supporting documents. Press and hold the calibration button located on the speedometer, start the vehicle and then release the button. Press that button again and then take the test drive. Once you have driven the distance needed, press the button once more and the speedometer will calibrate itself to accommodate the new tire size.
Now that you know how to change speedometer calibration, you can get those new tires you have been eyeing.