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SwissChap-
Are tyre widths as they are sold usually meant to be correct on a rim with 15mm internal width?
That seems to be about right for skinny road tyres, obviously not for 4" fat bike tyres. I expect there's some sort of table in the ETRTO standard relating tyre sizes to rim sizes. There are also tolerances, so if it says "25C tyre must be nominal size on 15C rim", that means something like "between 24mm and 26mm"
gbj_tester
23+4=27
Using your formula:
25+((21-15)/2)=28, but really my 25C GP4000S comes up to 30mm on a 21mm internal Mavic Crossride
25+((16.5-15)/2)=25.75, but really my 25C GP4000S comes up to 26.5mm on a 16.5mm internal Mavic MY2016+ Aksium
If I were trying to guess at a rule of thumb for estimating the width of a tyre on a rim bigger than the one on which it manifests its nominal width, I'd divide the difference by π/2 (about 1.6) rather than dividing by 2.
25+(2(21-15)/π)=29
25+(2(16.5-15)/π)=26
Because the extra width is actually a chord rather than an arc, this formula will always come up with an answer smaller than the correct result, but it will be closer than your version 🙂