How to use it
- Type the size printed on your current tyre into Current tyre size, for example 205/55 R16.
- Type the size you plan to fit into New tyre size, for example 225/45 R17.
- Read the Change in overall diameter. Michelin (UK) allows a diameter change only within -2 percent and +1.5 percent in some countries.
- Check What the speedometer shows: the table gives the reading at 50, 90 and 120 km/h and whether it passes the UN R39 rule.
How a tyre size is read
A metric size such as 205/55 R16 gives the section width in millimetres (205), the aspect ratio as a percentage of the width (55) and the rim diameter in inches (16). The sidewall height is the width times the aspect ratio: 205 × 55 ÷ 100 = 112.75 mm.
The overall diameter is the rim plus two sidewalls: 16 × 25.4 = 406.4 mm plus 2 × 112.75 mm, which gives 631.9 mm. An inch size such as 31x10.5R15 gives the overall diameter directly, here 31 × 25.4 = 787.4 mm.
Sidewall (mm) = width × aspect ratio ÷ 100
Overall diameter (mm) = rim (inches) × 25.4 + 2 × sidewall
Inch size: overall diameter (mm) = first number × 25.4What the speedometer does
The speedometer counts wheel turns and multiplies them by the circumference the car was set up for. A bigger tyre covers more distance per turn, so the car shows a lower speed than it really makes. A smaller tyre shows a higher speed.
UN Regulation 39 sets the accuracy. The speedometer must never show less than the true speed, and may show at most 10 percent plus 4 km/h more. At a true 50 km/h that is at most 59 km/h.
- The regulation applies to the car as approved. A different tyre size can break the rule, which is why the calculator marks the rows that fall below the true speed.
- Changes to tyre size are restricted: all four tyres should be changed at the same time, and only sizes the maker approves for the car are safe to fit.
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: 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 different size.
Worked example: 205/55 R16 to 225/45 R17
The current tyre is 631.9 mm overall. The new one has a sidewall of 225 × 45 ÷ 100 = 101.25 mm, so its overall diameter is 431.8 + 2 × 101.25 = 634.3 mm.
The change is (634.3 − 631.9) ÷ 631.9 = 0.38 %. At a true 50 km/h the speedometer shows 50 × 631.9 ÷ 634.3 = 49.81 km/h, an error of −0.38 %: it reads below the true speed, which UN R39 does not allow.
| Current 205/55 R16 | New 225/45 R17 | |
|---|---|---|
| Overall diameter (mm) | 631.9 | 634.3 |
| Revolutions per km | 503.7 | 501.8 |
Frequently asked questions
How do I read the size on my tyre?
The sidewall prints it, for example 205/55 R16 95V. The first number is the width in millimetres, the second the aspect ratio in per cent, and R16 the rim in inches. The letters and numbers after that are the load index and speed rating.
Why does a bigger tyre make the speedometer read low?
The speedometer counts wheel turns and assumes the circumference of the tyre it was set up for. A bigger tyre covers more distance per turn, so the car travels faster than it shows.
What is UN R39?
UN Regulation No. 39 is the international rule for speedometer accuracy. It says the speedometer must never show less than the true speed, and may show at most 10 percent plus 4 km/h more.
Can I fit a bigger tyre than my car came with?
Only a size the maker approves for your car, or a change within the limits your country allows. Michelin (UK) says some countries allow a diameter change only within -2 percent and +1.5 percent, and that all four tyres must be changed together.
Is the overall diameter the same as the rim size?
No. The overall diameter is the rim plus two sidewalls. For 205/55 R16 the rim is 406.4 mm and the overall diameter is 631.9 mm.
Are the figures exact?
They are theoretical, worked out from the size code. The real diameter of a tyre depends on the make, the load, the pressure and the wear, and the maker's data gives the exact figure.
Sources
- Wikipedia: Tire code (the ISO metric and inch formats and the overall diameter formula)
- UN Regulation No 39 (speedometers), 02 series, Official Journal of the EU L 2025/1902
- Michelin UK: Can you change your tyre's size?
Figures and text last checked on Tuesday, November 10, 2026.