How to use it
- Type the Original tyre size (the speedometer was set for), for example 195/65 R15, from the sidewall of the factory tyre.
- Type the New tyre size, the one fitted now, in the same format, for example 205/55 R16.
- Enter the Actual speed, km/h and the Distance on the odometer, km.
- Read Speedometer shows, km/h, Speedometer error, % and UN R39 check, then the tables of other speeds and odometer readings.
How a tyre change moves the speedometer
The speedometer and the odometer count the turns of a wheel and multiply them by the rolling circumference the car was set up for. A tyre of another diameter covers a different distance per turn, so both instruments read by the same share: the speed shown is the true speed times the original diameter divided by the new one.
The distance shown on the odometer is changed by the same share. A bigger tyre makes both read low, and a smaller tyre makes both read high.
Speedometer shows = actual speed × original diameter ÷ new diameter
True distance = odometer × new diameter ÷ original diameter
Error (%) = (original diameter ÷ new diameter − 1) × 100
Overall diameter (mm) = rim (inches) × 25.4 + 2 × width × aspect ÷ 100What UN R39 allows
- The speedometer must never show less than the true speed, and may show at most 10 percent plus 4 km/h more (clause 5.4). At a true 50 km/h the most it may show is 59 km/h.
- The total distance shown by the odometer may not deviate by more than 4.0 per cent from the true distance (clause 5.8).
- The rule applies to the car as approved, so a tyre size outside the approval can break it even when the car was fine before.
What this calculator does not include
- Load index and speed rating, which decide whether a tyre is suitable for your car.
- Tyre pressure and wear, which both change the real rolling diameter.
- The sizes approved for your vehicle. Check the vehicle documents or the maker's list before you fit a size.
Worked example: 195/65 R15 to 205/55 R16 at 100 km/h
The car was set up for 634.5 mm. The new tyre is 631.9 mm, so each kilometre takes more turns and the speedometer reads 634.5 ÷ 631.9 = 1.0041 times high: 100 km/h shows 100.4 km/h, an error of +0.41 %.
The odometer changes by the same share. After 10,000 km on the odometer the car has really covered 10,000 × 631.9 ÷ 634.5 = 9,959 km, so the odometer is 41 km too high. Both are well inside UN R39.
| Result | |
|---|---|
| Speedometer at 100 km/h | 100.4 km/h (+0.41 %) |
| Odometer at 10,000 km | 9,959 km really covered (41 km over) |
| UN R39 check | Within UN R39 |
Frequently asked questions
Why does a bigger tyre make the speedometer read low?
The speedometer counts wheel turns and multiplies them by the circumference it was set up for. A bigger tyre covers more distance in each turn, so the car travels faster than the speedometer shows.
Why does a smaller tyre make the speedometer read high?
Each turn covers less distance, so the wheel turns more often for each kilometre. The speedometer counts more turns than the distance covered, so it reads high by the share the diameter is smaller.
Does the odometer show the same error as the speedometer?
Yes, by the same share. Both count the same wheel turns, so a tyre that makes the speedometer read 1 per cent low also makes the odometer show 1 per cent less than was covered.
What is the UN R39 limit for the odometer?
UN Regulation No 39 says the total distance shown may not deviate by more than 4.0 per cent from the true distance (clause 5.8). The speedometer limit is separate: it may not show less than the true speed, and may show at most 10 per cent plus 4 km/h more (clause 5.4).
Can I fit a bigger tyre than my car came with?
Only a size the maker approves for your car, and within the limits your country allows. Check the vehicle documents or the maker's list before you fit a size, and fit all four tyres together.
Are the tyre diameters exact?
No. They are worked out from the size code, so they are theoretical. The real diameter depends on the make, the load, the pressure and the wear, and the tyre maker's data gives the exact figure.
Does the calculator keep the numbers I type?
No. The calculation runs in your browser, and nothing you type is sent to a server or saved for later.
Sources
- UN Regulation No 39 (speedometer and odometer equipment), 02 series, EUR-Lex 2025/1902, English text: clauses 5.4 and 5.8
- Wikipedia: Tire code (ISO metric and inch formats, the 82 % default, the Japanese diameter form and the overall diameter formula)
Figures and text last checked on Tuesday, November 10, 2026.