Tyre Size and Speedometer Error Calculator
Type your current and new tyre sizes as written on the sidewall (for example 205/55 R16, or an inch size such as 31x10.50 R15) to compare their sidewall height, overall diameter, circumference and revolutions per km and per mile. Add the speed your speedometer shows to see your true speed after the change, set beside the speedometer rule of UN Regulation No. 39 (used in the EU, UK, Japan and others).
This tool does not check fitment, load index, speed rating or clearance. Fit only tyre sizes approved by the vehicle maker or shown on the tyre placard. It gives no verdict on whether a change is allowed where you drive.
- Change in overall diameter
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- True speed at the speed shown
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- Speedometer reading compared with true speed
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- Speedometer reads
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- UN R39 approval range at this true speed
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- Overall diameter, current tyre
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- Overall diameter, new tyre
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Checked 3 November 2026Tested: 12 casesSources: UN Regulation No. 30, UN Regulation No. 54, UN Regulation No. 39 +1
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Assumptions (3)
- Sizes are the nominal sizes of the designation (UN R30), or the UN R54 table for listed inch sizes; real tyres vary a little with make, wear and pressure.
- True speed assumes the speedometer was exact on the current tyres; real speedometers are usually set to read a little high.
- The UN R39 range is the approval test for a new vehicle on its normally fitted tyres, shown for comparison only.
Show working
- Metric size W/A R d (UN R30 para. 6.1.2.1): sidewall height H = W × A ÷ 100; overall diameter D = d + 2H, with the rim code d × 25.4 mm.
- Inch size (e.g. 31x10.50 R15): D from the UN R54 Annex V table where the size is listed, otherwise the first number × 25.4 mm.
- Circumference = π × D; revolutions per km = 1,000,000 ÷ circumference in mm; per mile = 1,609,344 ÷ circumference in mm.
- True speed = speed shown × new D ÷ current D; speedometer reading compared with true speed = (shown − true) ÷ true × 100.
- UN R39 para. 5.4: 0 ≤ (V1 − V2) ≤ 0.1 V2 + 4 km/h, where V1 is the speed shown and V2 the true speed (4 km/h = 2.49 mph).
Worked example
Changing from 205/55 R16 to 225/45 R17 with the speedometer showing 100 km/h.
| You enter | |
|---|---|
| Current tyre size | 205/55 R16 |
| New tyre size | 225/45 R17 |
| Speed shown on the speedometer | 100 |
| Speed unit | km/h |
| You get | |
| Change in overall diameter | 0.38% |
| True speed at the speed shown | 100.4 km/h |
| Speedometer reading compared with true speed | -0.38% |
| Speedometer reads | 0.4 km/h below true speed |
| UN R39 approval range at this true speed | 100.4 to 114.4 km/h |
| Overall diameter, current tyre | 631.9 mm |
| Overall diameter, new tyre | 634.3 mm |
225/45 R17 is 0.38% larger in overall diameter than 205/55 R16 (634.3 mm against 631.9 mm). If the speedometer was exact on 205/55 R16, it shows 100 km/h when the true speed is 100.4 km/h.
Questions (4)
How do I read a tyre size such as 205/55 R16?
205 is the section width in mm, 55 the aspect ratio (the sidewall height as a percentage of the width), R a radial tyre, and 16 the rim diameter in inches. This is the designation in UN Regulation No. 30.
Why does a bigger tyre make the speedometer read low?
A larger tyre travels further each turn, so at the same wheel speed the car goes faster than the speedometer, set for the original tyres, shows. A smaller tyre does the opposite.
What does UN R39 say about speedometers?
In the approval test of a new vehicle, the speed shown must never be below the true speed and may be at most 10% plus 4 km/h above it. It is used in the EU, UK, Japan and others; other countries have their own rules.
What do sizes like 31x10.50 R15 mean?
An inch size: about 31 in overall diameter, 10.50 in section width, on a 15 in rim. Where UN R54 lists the size, its table diameter is used (775 mm for 31x10.50 R15), otherwise the nominal 31 in.
Sources and checks
More (3)
- UN Regulation No. 54 (tyres for commercial vehicles), Annex V, Table B: high flotation tyre sizes and outer diameters (OJ L 183, 11.7.2008, p. 41)
- UN Regulation No. 39 (speedometer and odometer), 01 series of amendments up to Supplement 1 (in force 10 October 2017), para. 5.4 (OJ L 302, 28.11.2018, p. 106)
- NIST Special Publication 811, Guide to the SI, Appendix B (inch and mile conversion factors)
Your numbers stay on your device unless you share them.
Results use the nominal tyre sizes. This tool does not check fitment, load index, speed rating or clearance, and gives no verdict on whether a change is allowed where you drive. Use only sizes approved by the vehicle maker or shown on the tyre placard, and ask a tyre professional before you change size. Results are estimates for information only and are not financial, tax or legal advice. Check important figures with a qualified professional before you act on them.
Both tyres side by side
| Current | New | Difference | |
|---|---|---|---|
| Size | 205/55 R16 | 225/45 R17 | |
| Sidewall height (mm) | 112.8 | 101.3 | -11.5 |
| Sidewall height (in) | 4.44 | 3.99 | -0.45 |
| Overall diameter (mm) | 631.9 | 634.3 | +2.4 |
| Overall diameter (in) | 24.88 | 24.97 | +0.09 |
| Circumference (mm) | 1,985.2 | 1,992.7 | +7.5 |
| Circumference (in) | 78.16 | 78.45 | +0.3 |
| Revolutions per km | 503.73 | 501.83 | -1.91 |
| Revolutions per mile | 810.68 | 807.61 | -3.07 |
| Diameter from | UN R30, 6.1.2.1 | UN R30, 6.1.2.1 |