Speedometer Calibration Calculator
Calculate speedometer error after changing tire diameter.
Loading calculator…
What this tool does
Calculate speedometer error after changing tire diameter. Assumes accurate original calibration and unchanged gearing.
How to use the Speedometer Calibration Calculator
- Enter or select original tire diameter in in.
- Enter or select new tire diameter in in.
- Enter or select indicated speed (mph).
- Read the calculated result; change any measurement to compare alternatives.
Formula
actual=indicated×newDiameter/originalDiameter; correction%=100×(newDiameter/originalDiameter−1)
- original
- Original tire diameter (in, before metric conversion)
- new
- New tire diameter (in, before metric conversion)
- speed
- Indicated speed (mph)
Assumes accurate original calibration and unchanged gearing.
Worked example
For speedometer calibration calculator, the following measurements illustrate the exact method: Original tire diameter: 26; New tire diameter: 28; Indicated speed (mph): 60.
Inputs
- Original tire diameter26 in
- New tire diameter28 in
- Indicated speed (mph)60
Result
- Actual speed (mph)64.62
- Correction (%)7.69
Results explained
- Actual speed (mph)
- Actual speed (mph) from the formula above. Assumes accurate original calibration and unchanged gearing.
- Correction (%)
- Correction (%) from the formula above. Assumes accurate original calibration and unchanged gearing.
Frequently asked questions
actual=indicated×newDiameter/originalDiameter; correction%=100×(newDiameter/originalDiameter−1)
Use the tire sidewall markings, actual vehicle specifications or recorded fuel/price values required by the inputs. Do not mix US gallons and imperial gallons.
Assumes accurate original calibration and unchanged gearing.
Yes. Change the relevant size, price or efficiency while keeping the other inputs constant. Results update immediately.
No. Check manufacturer fitment, load and speed ratings, clearance and local requirements with a qualified technician.