Tyre size confusion

Recently, I tried to swap tyres from my day-to-day road bike to a veteran bike I use only occasionally. It led me into a tyre sizing minefield. I checked your website, which told me that ISO 25-630 was a smaller diameter than 32-622. But when I tried to put a tyre from one bike, with an ISO stamp of 32-622, onto a wheel where the existing tyre was marked 25-630, it was too small. Am I missing something?

Pat Ryan

The tyre sizes page on our website states clearly that ‘overall diameter approximately equals the bead diameter plus twice the [tyre cross] section’. Therefore, a 25-630 (630mm + 50mm = 680mm) tyre has a smaller overall diameter than a 32-622 (622mm + 64mm = 686mm) tyre. The site also states: ‘the three-digit number after the dash… is the bead diameter at which the tyre fits onto the rim’. As 622mm is smaller than 630mm, a 622mm tyre cannot fit a 630mm rim (nor vice versa). Essentially, wheel diameter is twice the tyre cross-section plus the rim diameter. If you know the two latter, you can work out the former.

Richard Hallett

​​Cycle’s Technical Editor