this post was submitted on 31 Jul 2025
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ETRTO 37-451 is the designation of what rim size this tire will fit. Not necessarily the outer dimensions of the tire when mounted, which depends both on pressure and rim internal width.
https://en.m.wikipedia.org/wiki/European_Tyre_and_Rim_Technical_Organisation
Tire size specs are infuriating and confusing at best. But at least ETRTO will help you buy a tire that will fit your wheels.
This is surprising. Are you sure of that? I've always understood the first number to be the tire width and that has matched many tires I've used over the years, from Maxxis, Panaracer, Schwalbe, Conti, Vittoria, and others. This is also true according to Michelin:
Schwalbe agrees:
And on the traditional inch marking:
Even if you go by the traditional designation of 20 x 1 3/8 on the CST, the width should be about 35mm (1 3/8" ~= 35mm).
Of course any of the width dimensions are approximate depending on inflation and exact tire model but they've always matched within a couple of mm in my experience. 7mm differences is shocking to me.
To my understanding the diameter is bead diameter, not the outer tire diameter, which matches the rim. And then for width-to-rim compatibility, there are ranges where a range of ETRTO widths fits a range of rim widths, like this.