A 205/55 R16 tyre has a diameter of 631.9 mm. Changing to 225/45 R17 gives 634.3 mm, a difference of +0.38%, so your speedometer would show 119.5 km/h at a true 120 km/h. Keeping the overall diameter within about 2% of the original is the usual rule.
How do you read a tyre size?
A tyre marked 205/55 R16 91V means: 205 mm wide, a sidewall height of 55% of the width, radial construction, a 16-inch rim, load index 91 and speed rating V. The overall diameter is rim diameter plus two sidewalls: 16 × 25.4 + 2 × (205 × 0.55) = 631.9 mm. The circumference, the distance travelled per rotation, is that diameter times π, about 1,985 mm.
Why diameter matters
Your speedometer and odometer count wheel rotations, so a larger rolling circumference means you actually travel faster than indicated. Many modern cars also use wheel-speed sensors for ABS, ESP and cruise control, which are calibrated to the original size. A small mismatch, below 2%, is generally fine. Larger differences can affect speedometer accuracy, traction-control behaviour and even the APK test.
Equivalent sizes for a change of rim
| Original | Common alternative | Diameter change |
|---|---|---|
| 205/55 R16 | 225/45 R17 | +0.4% |
| 205/55 R16 | 215/50 R17 | +0.1% |
| 225/45 R17 | 235/40 R18 | +0.1% |
| 195/65 R15 | 205/55 R16 | −1.4% |
What else to check before you buy
- Load index and speed rating must be at least what your registration papers list.
- Wheel arch clearance. A wider tyre can rub at full lock or over bumps.
- Rim width. Each tyre width has an approved rim range. Your supplier can confirm the match.
- Cost and fuel use. Wider tyres and low-profile sizes usually cost more and ride harder, which matters if you are comparing the total cost of car ownership in the private lease calculator.
Winter tyres are often fitted in a smaller diameter, which is a good use of this calculator. If you are comparing a car’s fuel use and kilometre cost, the operational lease calculator can show the cost per kilometre.
Speedometer error in practice
Speedometers are not perfectly accurate even with the original tyres. By regulation they may never show less than the real speed and may show up to 10% plus 4 km/h more, so many cars read about 2% to 5% high. A tyre with a 1% larger circumference narrows that gap a little, while a smaller tyre widens it. At 120 km/h, a 3% error is 3.6 km/h, which matters mostly if you rely on the speedometer around speed cameras and in 100 or 130 km/h zones. A GPS app gives a good reference for the real speed.
The odometer scales the same way, so a larger tyre means you travel more real kilometres than the car records. Over 20,000 km a year, a 2% difference is 400 km, small but relevant for lease contracts based on registered mileage.
How to use this calculator
- Read your current tyre size on the sidewall, for example 205/55 R16.
- Enter the width, aspect ratio and rim size, then the same for the new size.
- Check the circumference difference and the speedometer error.
- Verify the load index, speed rating and approval for your car before buying.
Key figures at a glance
| Diameter formula | rim × 25.4 + 2 × width × ratio ÷ 100 (mm) |
|---|---|
| Common tolerance | About ±2% of original diameter |
| Check | Load index, speed rating, approval |
Frequently asked questions
Multiply the rim size in inches by 25.4, then add twice the sidewall height, where sidewall height is width times the aspect ratio divided by 100.
A common guideline is up to 2%, but check your car’s approved sizes and the APK rules. Always verify with a tyre specialist.
Yes. A larger diameter means the speedometer reads lower than your true speed. A smaller diameter means it reads higher.
The sidewall height is 55% of the 205 mm tread width, so 112.75 mm.
Only within the approved rim width range for that tyre. Check the tyre manufacturer’s rim-width guide.