Speedometer Error Calculator
Compare original and replacement metric tire sizes to estimate how indicated speed changes.
How to use this calculator
- Enter the original tire size that the vehicle's calibration is based on and the proposed replacement size.
- Enter an indicated speed to see the geometric estimate of actual road speed if calibration remains unchanged.
- Check the percentage diameter change as well as the example speed difference. The percentage relationship applies at other speeds in the same simple model.
- Use nominal dimensions for planning only. Actual tire circumference varies by make, model, inflation, load and wear, and modern vehicle systems may use more than simple wheel revolutions.
- Do not use a small speedometer difference as proof that the replacement tire is acceptable. Fitment, load capacity, braking and stability-system requirements must be checked separately.
Formula and assumptions
Estimated actual speed = indicated speed × new nominal diameter ÷ old nominal diameter. Percentage speed change is the same as the percentage change in nominal circumference because circumference is directly proportional to diameter.
GIIRR calculates idealized geometry and kinematics. Friction, slip, compliance, tire deformation, manufacturing tolerances and control-system behavior can move real measurements away from the theoretical result.
Worked example
If the original tire has a nominal diameter of 24.97 inches and a replacement is 25.33 inches, the new tire is about 1.4% larger. With unchanged calibration, an indicated 60 mph would correspond to roughly 60.9 mph in the simple geometric model. The same percentage also changes effective gearing, so engine RPM at a fixed true road speed would be slightly lower.
Frequently asked questions
Is a small diameter change always acceptable?
No. Clearance, wheel compatibility, load and speed ratings, ABS/ESC behavior and manufacturer limits also matter.
Does this recalibrate the vehicle?
No. It estimates the geometric effect only. Actual recalibration procedures depend on the vehicle and its electronic systems.
Why might GPS show a different error than the calculator?
The calculator uses nominal tire geometry. Real rolling circumference, factory speedometer bias, tire wear and calibration logic can make the observed difference larger or smaller.
Does a larger tire make the speedometer read low or high?
In the simple unchanged-calibration model, a larger tire travels farther per wheel revolution, so actual road speed is higher than the indicated value derived from the original smaller tire.
Does tire size also affect engine RPM?
Yes. A larger rolling circumference acts like slightly taller effective gearing, reducing wheel revolutions and therefore engine RPM at the same true road speed when mechanical ratios are unchanged.
For safety-critical, legal or fitment decisions, verify the result against manufacturer documentation and physical measurements.
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