Tyre Sizes Explained

Over the years cycle tyre sizes seem to have become one of the most confusing aspects of cycling. It can actually be really easy to buy the wrong tyres for your bike. Even more so if you’re doing anything other than replacing old tyres with the exact same make / model.

Tyre Markings

Markings on the tyre aren’t always going to be of much help. Sometimes tyres that look different can be the same, and some that appear to be the same are different. Much of this comes from the differences between the traditional Imperial and American systems of defining tyres sizes. If you look closely at the chart at the end of this article you will notice that 27 x 1 1/4 (imp) and 27 x 1 3/8 (U.S.) are actually the same size. Conversely 26 x 1 1/2 (imp) and 26 x 1.5 (U.S.) are actually completely different! When you add in to the mix the Metric system it becomes even more confusing

Marking the tyres with the International Standards Organisation (ISO) system, alongside any other legacy marking goes some way to removing this ambiguity.

The ISO system uses two sets of numbers separated with a dash (such as 28-622). The first number is the sectional width of the tyre and is approximately the same as the tyre width. In practice it’s usually a little less and can vary according to the rim on which it’s fitted. The height of the tyre and it’s section are usually pretty close which helps to calculate tyre diameter and thus circumference.

The three numbers after the dash are the bead diameter for where the tyre fits on the rim. This is important in making sure the tyres fit on the wheel and is probably more important that the width in making sure that a tyre will fit correctly.  So, to use the example above any tyre with a ‘622’ bead diameter will fit any wheel of that size. (Note ‘622’ is about the most common size of wheel around at the moment, and the reason we’ve used it as the example in this article).

Wheel / Tyre compatibility

In essence any tyre with the same ISO designation will work on the same wheel, even if their alternate markings appear to be contradictory. As with tyres, marking wheel rims with their ISO designaion has become common. This should make selecting the right tyre that much easier.

The ISO system is universally used on tubeless tyres. In general they follow the sizings in the table. Using tyres and rims that are made for use with tubeless will get the best sealing effect.

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