Speedometer Error Calculator
True speed after a tire or gearing change.
Enter your values
ⓘ Live results
Results
- True speed
- 73.7 mph
- Error
- 5.26% reading low
- Odometer per 100 indicated miles
- 105.3 real miles
- Speed at indicated 30 mph
- 31.6 mph
Quick answer
Speedometer error is the ratio of current tire diameter to stock diameter. Taller tires make the needle read low; shorter tires make it read high.
True speed = indicated × (new diameter ÷ stock diameter)
How to use the Speedometer Error Calculator
- 1Enter the stock tire diameter from the door placard size.
- 2Enter the diameter of the tire fitted now.
- 3Enter any indicated speed to see the true speed.
Worked example
Using the values the calculator loads with:
Inputs
- Stock tire diameter28.5 in
- Current tire diameter30.0 in
- Indicated speed70 mph
Results
- True speed73.7 mph
- Error5.26% reading low
- Odometer per 100 indicated miles105.3 real miles
- Speed at indicated 30 mph31.6 mph
Frequently asked questions
Can I recalibrate the speedometer?
Most modern vehicles allow a tire-size recalibration with a scan tool or dealer software; older vehicles may need a speedometer gear or an inline calibrator.
Is a speedometer allowed to read high?
In most regions a speedometer may over-read but never under-read, which is why factory needles usually sit a few percent optimistic.
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.