For speedometer calibration, there are a few things to know to get the best possible results. Below we will cover what you need to know in detail. There is no easier or more accurate method than ECRI's GPS Speedometer Calibration, covered at the bottom of the post.
If you want a fast and pretty close speedometer calibration, program your tire size one inch shorter than the advertised tire size. This will normally be extremely close. If you want more information about calibrating your speedometer, keep with us.
Example of a tire spec chart showing the overall diameter when measured at max PSI on a certain width rim without vehicle weight applied
The most common error in calibrating a speedometer is using the size stamped on the sidewall of a tire. This is not correct for the follow reasons:
All of these factors combine to create a rolling diameter that can be much smaller than the advertised tire size.
The easiest method for measuring your rolling diameter is to measure from the center of your hub/wheel to the ground, then multiply by 2. This will account for manufacturing variances and sidewall deflection.
Vehicle manufacturers build a speedometer tolerance into vehicles to account for changing tire pressures, tire wear and manufacturers discrepancies for the original size tire. A normal speedometer tolerance is 3%. This translates to a speedometer that reads approximately 2mph slow at 60mph.
The odometer will normally not be affected by the speedometer tolerance, which means if you drove with your speedometer reading 100mph for an hour, your odometer would be at 103 miles. This is a small detail that shouldn’t matter to most people, but we’ve had customers notice before.
We've covered a lot of variables and math...which might be too much too think about. The easiest way to calibrate your tire size is to use the GPS feature in the ECRI app. This feature will compare readings from your Jeep and phone’s GPS to determine the correct tire size setting for you. Here are some tips for the results:
