Q: I recently had a new set of tires installed. I opted to go with a larger diameter than the originals that were on the car. Will this affect the accuracy of my speedometer? How do I check?
A: There is a good chance you need to recalibrate your speedometer whenever you’ve changed tire size or altered your gear ratio. One tried and true way to gauge the accuracy of your speedometer is to find a stretch of highway with mile markers. Travel at a constant speed and see how many seconds it takes to go one mile at 55 miles-per-hour. It should take 65 seconds. At 65 miles-per-hour, it should take 55 seconds to go that same mile. Repeat this test a few times to get an accurate average. If the average is way off the times above, you will need to recalibrate your speedometer to make sure your engine computer has accurate performance data–and to stay on the good side of the law.
You can find a wide range of speedometer recalibration devices to help you get your speedometer dialed in to your new tires or any other upgrades that might affect its accuracy.