Tyre Size Calculator
Type the size from the sidewall, such as 205/55 R16. Add a second size to see the change in diameter and what your speedometer will read.
This is an estimate based on the printed size. The real diameter of a tyre varies a little between makes.
Worked examples
Moving up a size
205/55 R16 changed to 225/45 R17
Diameter 631.9 mm to 634.3 mm, a rise of 0.38%
A dial reading of 70 mph is a real 70.3 mph.
About Tyre Size Calculator
A tyre size calculator decodes the numbers on the sidewall. In 205/55 R16, the 205 is the width of the tyre in millimetres, the 55 is the height of the sidewall as a percentage of the width, and the 16 is the diameter of the wheel in inches. From these the tool works out the sidewall height, the overall diameter of the tyre and its circumference, which is how far the car moves in one turn.
If you fit a different size, the overall diameter changes, and so does your speedometer, which counts wheel turns. A taller tyre means the car goes faster than the dial says. Enter both sizes to see the change in diameter and the real speed at 30, 50, 60 and 70 mph. Most makers allow a change of only a few per cent, so check the handbook and with a tyre fitter before you change.
How to use Tyre Size Calculator
- Type the size of the tyre as printed on the sidewall.
- Type a second size to compare.
- Check the change in diameter.
- Read the real speeds for the speedometer readings.
Common uses
- Fitting larger alloy wheels
- Choosing winter tyres
- Checking a speedometer error
- Comparing tyre sizes before you buy
Frequently asked questions
What does 205/55 R16 mean?
205 is the tread width in mm, 55 means the sidewall is 55% of 205 mm (112.75 mm), and 16 is the rim diameter in inches. The R stands for radial construction.
How big is a 205/55 R16 tyre?
The overall diameter is 631.9 mm and the circumference is about 1,985 mm, so the wheel turns about 811 times a mile.
What happens if I fit 225/45 R17 instead?
The diameter becomes 634.3 mm, 0.38% bigger. When the dial says 70 mph you are doing about 70.3 mph, a negligible change.
How much difference is acceptable?
The overall diameter should normally be within about 3% of the original. A bigger change can upset the speedometer, ABS and traction systems and gearing.
Do load and speed ratings matter?
Yes. The numbers after the size, such as 91V, give the load and speed rating, and new tyres must be at least as high as the manufacturer's.