In order for ECRI to give you the most accurate speedometer reading possible; here are some tips we highly recommend using for accurately measuring your tire size before recalibrating your vehicle.Locate the center of your hub/wheelFrom your hub measure to the ground.Multiply that value by 2X = Center of the hub to the groundX * 2 = Rolling diameter that we need to use for calibrationThe reason we need to measure this way is to account for a couple different factors that will make the tire size shorter than the size stamped on the tire. First: most tire manufacturers make tires that are slightly shorter than the advertised size. Second: the weight of the vehicle on the tire will compress the tire to make it shorter still. Overall, we’ve found that the rolling diameter is usually around an inch shorter than the size stamped on the sidewall of most tires, which would mean a speedometer that is off by about 3-percent!