Tire Diameter Speedometer Error Calculator
How a tire size change affects speedometer accuracy.
Enter your values
ⓘ Live results
Results
- Speedometer error
- 5.96%
- Actual speed when showing 70 mph
- 74.2 mph
- Direction of error
- Speedometer reads slow
- Diameter change
- 1.7 in
Quick answer
Speedometer error percentage equals the percentage change in overall tire diameter between old and new tires.
error % = (new diameter - old diameter) / old diameter x 100
How to use the Tire Diameter Speedometer Error Calculator
- 1Enter the original tire's overall diameter.
- 2Enter the new tire's overall diameter.
- 3The error percentage shows how far off your speedometer and odometer will be.
Worked example
Using the values the calculator loads with:
Inputs
- Original tire diameter28.5 in
- New tire diameter30.2 in
- Indicated speed70 mph
Results
- Speedometer error5.96%
- Actual speed when showing 70 mph74.2 mph
- Direction of errorSpeedometer reads slow
- Diameter change1.7 in
Frequently asked questions
Why does bigger diameter make the speedometer read low?
A larger tire covers more distance per revolution, so the speedometer (calibrated for the old size) underreports actual speed.
Can this be recalibrated?
Many modern vehicles allow speedometer recalibration through the ECU or a gear ratio adjustment.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.