How Do I check Speedometer Accuracy?

Installing tires that are larger or smaller than what came O.E. (original equipment) is often an attractive way to enhance performance or style. However, most vehicles' speedometers measure speed based on tire revolutions - calculated from the original tires' size. So, changing the tires' diameter will affect speedometer accuracy.

Up-sizing to taller tires, with a larger OD, will result in tires that are traveling more distance per revolution than the O.E. tires, for a slower actual speedometer reading. Downsizing to shorter tires, with a smaller diameter, equals less distance traveled per rotation, for a faster speedometer reading. To sum it up; Taller = Slower, Shorter = Faster!

So how fast are you traveling? Using the chart below, follow the instructions to determine your speedometer's accuracy.

While traveling at a constant speed on an open highway, have a passenger record the number of seconds it takes to travel one measured mile, from one mile marker to the next.

On the chart, locate the number of seconds it took to travel one mile. The speed indicated next to the number of seconds is the true road speed of your vehicle.

Repeat the measurement at least three times and average the result. Recalibration of your speedometer is recommended if a significant inaccuracy is noted. Drive safely!