Speedometer Calibration CalculatorSpeedometer Error & Correction Factor for New Tires
Speedometer error (%) = (new tire diameter ÷ original diameter − 1) × 100, and true speed = indicated speed × new diameter ÷ original diameter.
Changed your tire size or checked your speed against GPS? Get your true speed, percentage error and the correction factor your tuner or programmer needs — plus how far your odometer drifts every 1,000 miles.
Calibration source
Metric, flotation (33×12.50R17) or diameter in inches.
New tire is 32.71 in tall vs. 31.61 in original.
Speedometer reads LOWIndicated vs true speed
True speed table
| Speedometer shows | True speed | Difference |
|---|---|---|
| 20 mph | 20.7 mph | +0.7 |
| 30 mph | 31.0 mph | +1.0 |
| 45 mph | 46.6 mph | +1.6 |
| 55 mph | 56.9 mph | +1.9 |
| 65 mph | 67.3 mph | +2.3 |
| 70 mph | 72.4 mph | +2.4 |
| 80 mph | 82.8 mph | +2.8 |
How do you calculate speedometer error after changing tire size?
Speedometer error (%) = (new tire diameter ÷ original diameter − 1) × 100, and true speed = indicated speed × new diameter ÷ original diameter. Going from 265/70R17 (31.61 in) to 285/70R17 (32.71 in) gives a correction factor of 1.0349: the speedometer reads 3.49% low, so an indicated 60 mph is really about 62.1 mph.
Part of our Tire, Wheel & Gearing calculators — for the core estimate, start with the Tire Size Calculator.
Two ways to calibrate your speedometer reading
Tire size method
Enter your original (factory) size and the new size. The ratio of diameters is the correction factor.
GPS method
Drive at a steady indicated speed on a flat road and read your phone or GPS speed.
Read the error
Positive error means the speedometer reads low; negative means it reads high.
Recalibrate
Use the correction factor or new revs per mile in a tuner, programmer or dealer tool.
Speedometer correction formula
True speed = speedometer reading × correction factor
Error % = (correction factor − 1) × 100
Odometer: actual miles = recorded miles × correction factor
GPS method: correction factor = GPS speed ÷ speedometer reading
Tire diameters come from the same math as our Tire Size Calculator, which also lists equivalent sizes within 3%.
Example: 265/70R17 to 285/70R17
| Indicated | True speed | Difference |
|---|---|---|
| 30 mph | 31.0 mph | +1.0 mph |
| 45 mph | 46.6 mph | +1.6 mph |
| 55 mph | 56.9 mph | +1.9 mph |
| 65 mph | 67.3 mph | +2.3 mph |
| 75 mph | 77.6 mph | +2.6 mph |
Correction factor 1.0349 (3.49% low). The odometer under-records about 349 miles every 10,000. These match the calculator’s defaults.
How accurate are speedometers from the factory?
Many manufacturers set speedometers to read slightly high rather than low, so drivers aren’t unknowingly speeding. In Europe and the UK, regulations (UN ECE Regulation 39) prohibit speedometers from showing less than the true speed. Worn tires, tire pressure and even different brands of the same size change rolling diameter slightly, so a small error of 1–2% is normal.
- Worn tread reduces diameter a little, so speedometers read slightly higher as tires wear.
- Under-inflation lowers the effective rolling radius.
- GPS speed is most reliable at a steady speed on a straight, level road.
How to recalibrate your speedometer
- Handheld programmers and tuners for many trucks and SUVs let you enter a new tire size or revs per mile.
- Dealer service tools can update the tire-size setting on many late-model vehicles.
- Inline calibration modules adjust the vehicle speed signal on older vehicles.
- Gear changes: if you also regear the axle to match bigger tires, enter the effective change; our Gear Ratio Calculator helps.
If you regear at the same time, the Gear Ratio Calculator shows the axle ratio that restores your original RPM.
Related calculators
Speedometer Calibration Calculator FAQs
Divide the new tire’s diameter by the original’s. Subtract 1 and multiply by 100 for the percentage. A positive number means the speedometer reads low.
Slower. With taller tires the speedometer shows a lower speed than you’re actually traveling.
Rules vary by country. In the EU and UK, UN ECE Regulation 39 says a speedometer must never show less than true speed. In the US, accuracy rules apply mainly to manufacturers and commercial vehicles; drivers are responsible for obeying limits regardless of gauge error.
Yes. Hold a steady speed on a straight, level road and compare the GPS reading. Divide the GPS speed by the speedometer reading to get the correction factor.
Yes, by the same percentage. With 3.5% taller tires, the odometer under-records about 35 miles every 1,000.
Use a tuner or programmer that supports tire-size changes, a dealer service tool, or an inline calibration module, depending on the vehicle.
The ratio of true speed to indicated speed — new tire diameter divided by original diameter. Multiply any speedometer reading by it to get your true speed.
About the developer
Manzoor Ahmad
Web Designer & Calculator Developer · 13+ years of web development and SEO experienceI’m a professional pharmacist turned tech geek, and I designed and coded this speedometer calibration calculator myself. It uses the same tire-diameter math as our Tire Size Calculator, so the two tools always agree.
Found a number that looks wrong, or want a feature added? Tell me through my author page — corrections are fixed and re-dated.
Sources
- UNECE — Regulation No. 39: speedometer and odometer
- NHTSA — Tires
- eCFR — 49 CFR Part 393 (commercial vehicle equipment)
Disclaimer: Results use nominal tire dimensions or your GPS reading. Actual rolling diameter varies with brand, wear and pressure. Always obey posted limits; recalibration procedures depend on your vehicle.
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