Originally Posted by bikera

Not true: the wider the rim the wider a given tire will be.

ETRTO sets the standards for tire/rim sizing designations. The ETRTO sizing standards say that the number in front of a tire’s “c” size should match the designated width on a 19c rim, and that the width should increase 1mm for each additional 2mm in rim I.W.

Hence a 40c tire should be 43mm on my 25mm I.W. rims. Most I’ve tried are really 42-ish (close enough) but many others (Conti, Pirelli, Panaracer, Schwalbe, Tufo, WTB…) should be calling their 40c’s a 37 or 38c (why they don’t who knows…the new version of this standard has been around for several years so they’ve had plenty of time to deal with it). Funny thing is that I’m only seeing this w/ gravel tires…the road tires I’ve tried recently seem to be more like just +/-1mm from ETRTO.