Tyre size calculator
A tyre size like 205/55 R16 means 205 mm wide, a sidewall 55% of the width, on a 16-inch wheel. Changing size changes the overall diameter, which affects your speedometer.
- Real speed
- 60.2 mph
- Current diameter
- 632 mm
- New diameter
- 634 mm
- Current sidewall
- 113 mm
- New sidewall
- 101 mm
205/55 R16 compared with 225/45 R17. When your speedometer shows 60 mph, you'd really be doing about 60.2 mph.
Check your car’s handbook or a tyre fitter before changing size, and tell your insurer about non-standard wheels.
Common UK tyre sizes
| Tyre size | Sidewall | Overall diameter |
|---|---|---|
| 175/65 R14 | 114 mm | 583 mm |
| 195/65 R15 | 127 mm | 635 mm |
| 205/55 R16 | 113 mm | 632 mm |
| 225/45 R17 | 101 mm | 634 mm |
| 225/40 R18 | 90 mm | 637 mm |
| 235/55 R18 | 129 mm | 716 mm |
How we calculate this
The sidewall height is the width multiplied by the profile percentage. The overall diameter is the wheel size (converted from inches to millimetres) plus two sidewalls.
A bigger diameter means each turn of the wheel covers more ground, so you go slightly faster than your speedometer shows. A smaller one does the opposite.
Questions people ask
What do the numbers on a tyre mean?
In 205/55 R16, 205 is the width in millimetres, 55 is the sidewall height as a percentage of the width, R means radial, and 16 is the wheel size in inches. There are usually load and speed ratings after it too.
Can I fit a different tyre size?
Sometimes. Many cars are approved for several sizes, listed in the handbook or on a sticker in the door frame. Keep the overall diameter close to the original, and check with a tyre fitter.
Do I need to tell my insurer?
Usually yes, if you change to wheels or tyres that are not a standard option for your car. Not telling them could affect a claim.
Related calculators
- Stopping distance calculatorHighway Code stopping distances at any speed, in dry, wet or icy conditions.
- Journey fuel cost calculatorWhat a car journey costs in fuel, split between passengers if you like.
- Electric vs petrol car running costYearly fuel cost of an electric car against petrol or diesel, at your mileage.
- EV charging time calculatorHow long an electric car takes to charge, on a 3-pin plug, wallbox or rapid charger.
- Mileage allowance calculatorHMRC approved mileage rates for 2026 to 2027: what you can be paid tax-free, or claim back.
- Car tax calculatorHow much vehicle tax (VED) a car costs a year, by CO2 and age.
Save or share this calculator
Embed this calculator
Free to use on your website, blog or newsletter. Copy this code and paste it into your page.