Tyre Size Calculator
Compare two tyre sizes and see the difference in sidewall height, overall diameter, circumference and estimated speedometer reading. For comparison and general information only — always check your vehicle handbook and get professional advice before changing tyre size.
Enter Your Tyre Sizes
| Measurement | Current | New |
|---|---|---|
| Sidewall height | – | – |
| Overall diameter | – | – |
| Circumference | – | – |
| Revolutions per mile | – | – |
| Revolutions per km | – | – |
How Tyre Size Numbers Work
The code stamped on a tyre's sidewall — something like 205/55R16 — describes three separate measurements. The first number is the tyre's width in millimetres, measured sidewall to sidewall. The second is the aspect ratio, the sidewall's height expressed as a percentage of that width. The letter confirms the tyre's construction type. The final number is the diameter, in inches, of the wheel rim the tyre is built to fit.
Why Overall Diameter Matters
Change any one of those three figures and the tyre's overall diameter changes too. A vehicle's speedometer and odometer are calibrated around the original tyre's rolling circumference — how far the wheel travels in one full turn — so fitting a noticeably larger or smaller tyre can make the displayed speed and distance drift from the actual figures. This calculator converts width, aspect ratio and rim size into an overall diameter and circumference for each tyre, so the two can be compared directly rather than guessed at.
A Note On Tolerance
Some in the industry treat a small percentage change in overall diameter — commonly cited as roughly 2–3% — as a rough guide for how close an alternative size stays to the original. This is not a legal limit, a manufacturer approval, or a guarantee that a given size will fit or perform correctly on your specific vehicle. Different vehicles tolerate different amounts of change, and this calculator does not label any result as "safe," "approved" or "compatible" — it only shows the numbers.
Other Factors This Calculator Doesn't Cover
Overall diameter is only one part of whether an alternative tyre size will actually work. Load index, speed rating, rim width compatibility, clearance around the wheel arch and suspension, tyre pressure monitoring system (TPMS) compatibility, and the effect on braking and handling can all matter too — and none of them are calculated here. A tyre size change can also have insurance and legal implications depending on what's fitted and how it's recorded.
Getting It Right
Before changing tyre size, check your vehicle handbook or the plate inside the driver's door for the manufacturer's approved sizes, and get advice from a qualified fitter who can confirm a size against your specific make and model. See our tyre range for what we carry, or our tyre replacement service to arrange a fitting — tell us your registration or current tyre size and we'll talk through suitable options.