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.
Too Long/Don’t want to read??
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.
Don't Trust the Sidewall
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:
- Tire manufacturers are allowed to make tires that vary from the advertised size...and they usually do.
- Tire manufacturers publish spec charts with the Overall Diameter listed, but this measurement is taken at max PSI without vehicle weight applied.
- Sidewalls deflect under load, which makes the tire shorter once installed on a vehicle.
- Tread wears as you drive, making the tire shorter over time.
- The tire pressure effects sidewall deflection and weather can effect tire pressure.
All of these factors combine to create a rolling diameter that can be much smaller than the advertised tire size.
Measuring Rolling Diameter
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.
Speedometer Tolerance
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.
Compensating for Speedometer Tolerance
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2018+ Jeep Wrangler JL and Gladiator JT
The new Jeeps have a speedometer tolerance setting that we can adjust. Simply change this setting at the same time as changing your tire size setting to your liking -
Other Vehicles (including 2007-18 Jeep Wrangler JK):
On vehicles that don't have a separate Speedometer Tolerance setting, you can compensate within the tire size setting. After getting the rolling diameter, multiplying by 0.97 will compensate for the speedometer tolerance. Keep in mind, not all vehicles have the same tolerance setting, so double check your speedometer using a GPS.
GPS Calibration
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:
- Have a passenger watch your phone while you drive.
- Drive a constant (legal) highway speed for at least a couple miles.
- The white arrows on the app indicate the range of 95% confidence for the tire size calculation. The closer the arrows are to each other, the more accurate the calculation.
- If you don’t have a convenient spot to stop and program, screenshot the calculated size, then program manually when it’s convenient.
