Tyre size comparison
Thinking of changing size? Enter both and we will show you the difference in diameter, how far out your speedometer would read, and whether the swap is within safe tolerance.
Common questions
How much can I change my tyre size by?
The accepted tolerance is ±3% on overall diameter. Inside that, your speedometer, odometer and driver aids stay accurate enough. Beyond it the speedometer reads incorrectly, ABS and traction control reference the wrong rolling radius, and the tyre may foul the arch.
Does a bigger tyre make my speedometer wrong?
Yes. A larger tyre covers more ground per revolution, and the speedometer counts revolutions — so it under-reads. You are travelling faster than the dial shows, which is the direction that earns a speeding fine rather than the one that protects you from it.
What is plus-sizing?
Fitting a larger-diameter wheel with a lower-profile tyre so the overall diameter stays about the same. Going from 225/45R17 to 245/40R18 is a textbook plus-one. It sharpens steering response but gives a firmer ride and far less protection against pothole damage.
Can I fit wider tyres on the same wheels?
Within limits. Each rim width supports a range of tyre widths, usually about 20mm either side of the original. Going too wide makes the tread crown and wear in the centre; too narrow stretches the tyre and stresses the bead. Check the diameter change here first, then ask us about rim width.