Free tool
Rolling radius calculator
Put your tyre size in and see what else is close enough to fit — how much taller or shorter each option is, and what it does to your speedometer.
Sizes you could use instead
| Size | Diameter | Difference | At an indicated 70 |
|---|
Nothing else comes within 3% of that size on the same wheel. Untick the box above to see sizes that need a different wheel.
Before you buy on the strength of this
- Diameter isn’t the only number that matters. The replacement must also carry at least the load index and speed rating on your original tyre — those are the two- or three-character codes after the size, like 91V. Going below either is an MOT failure and an insurance problem.
- Check the tyre suits your wheel width. A much wider or narrower tyre can work out to the same diameter while being wrong for the rim it has to sit on.
- Both tyres on an axle should match. Different sizes across an axle change how the car brakes and steers.
- On a 4x4 or all-wheel-drive car, all four need to match. Mismatched rolling radius on an AWD car wears the centre differential or coupling out — see replacing tyres on a 4x4.
- The manufacturer’s list wins. Your handbook, or the plate in the driver’s door shut, lists the sizes approved for your car. Anything else is at your own risk, and a size change can be a declarable modification on some policies.
These are nominal figures worked out from the size itself. Real tyres vary by a few millimetres between makes, and a loaded tyre sits slightly shorter than a free one — so use this to narrow the choice, then check the maker’s own figure for the exact tyre.