Q: “I just got new rims and tires. It was an upgrade to new alloy wheels with thin performance tires. I can tell that this has changed the accuracy of my speedometer because it is registering a different speed than my GPS does. Is there a way to calculate how far off it is and recalibrate my speedometer so it is correct again?”
Signed – Speeding Out of Control in Victoria, BC
This is a great question, and a common one. When people move to nicer rims and lower profile tires we often see the total wheel height decreasing, which would mean that the speedometer is showing a faster speed than the vehicle is actually going. On the other side of the spectrum, when people move to bigger tires to improve the smoothness of their ride, the opposite effect on the speedometer occurs. You can see the effect demonstrated in this animation:
Simply put, as the size of your wheel changes, so does the distance it travels in one revolution. Since the speedometer is registering the number of revolutions, we get a disparity in the number displayed on the speedo. In the old days this was actually quite easy to fix. Since the speedometer gear was mechanical you would change its gears up or down to compensate for the height change. In modern vehicles they are no longer mechanical, which means they require a tweak to the engine computer to change the internal wheel height measurement. This is not possible in all vehicles and is completely dependant on the make, model, and year of your vehicle. I would wager a guess that about 25% of vehicles on the road have accessible wheel height adjustments using aftermarket engine computer scanners. Many GM and Dodge vehicles can be interrogated and changed with our scanners, as can some others, depending on the year.
Something to Consider
If your wheel size has changed drastically you should take special note of how much it has changed before you do any highway driving. As you speed up, the disparity between your actual speed and the speedometer readout will get larger and larger exponentially. You can test your true speed quite accurately if you have a GPS, as many will give you your current speed. You can compare the numbers and determine the speed difference by percentage so you can do some simple math in your head while you are driving. Always remember that an incorrect speedometer caused by an aftermarket tire change is your responsibility, so you will be at fault if the police issue you a ticket.
Hypertech has a speedometer calibrator for personal use that you connect to your engine computer to make the necessary wheel height adjustments. There are a few caveats, though, as the devices are vehicle make and model specific, so you would need a different device for each vehicle you would like to calibrate. Not all makes are supported, for the reasons listed above, and the price might be prohibitive. To change the ride height on a Cadillac Escalade, for example, would cost $239USD, which is a lot for a one time adjustment. You may find you are better off choosing a rim and tire combo that matches your current ride height instead of choosing aftermarket computer calibration.