Free tool
Tyre size calculator
Pick the size on your sidewall and every number appears instantly — sidewall height, overall diameter, circumference, revolutions per mile, speedometer error and the alternative sizes that keep your car honest.
Step one
Your current tyre
No need to know the jargon — pick each number and watch it appear on the tyre exactly where it is moulded on yours.
Choose a width, profile and rim and the maths appears instantly.
Sizes we stock today
Every size below has tyres in the catalogue right now, each with its own fitted-price page.
The rules behind the calculator
What counts as a safe alternative tyre size?
The accepted convention is that an alternative size should keep the overall rolling diameter within 3% of the original fitment. Inside that envelope the speedometer, gearing and arch clearance stay close to what the manufacturer intended; outside it, stay with your original size.
Why is a taller tyre riskier than a smaller one?
UK law requires a speedometer never to show less than the car's true speed, while it may over-read by up to 10% plus 6.25 mph. A smaller tyre makes the speedometer over-read, which stays legal; a taller tyre pushes it towards under-reading, so the taller direction is the one to treat with caution.
How do I read a tyre size like 225/45 R17?
The first number is the tread width in millimetres, the second is the sidewall height as a percentage of that width, and the figure after the R is the wheel diameter in inches. The load index and speed rating that follow — 94Y, for example — are moulded into the sidewall too. Read the full guide to tyre size markings.