Originally Posted by
Iride01
I don't know exactly where I got that. However it was from someone that writes technical articles such as Leonard Zinn. However I don't necessarily think it was any of his articles as I was reading articles from a lot of writers that dealt with things mechanical on bicycles back then. And it was at least 15 or 20 years ago I read that.
Perhaps it's just the way some manufactures measure it.
Regardless, if the tire measures bigger, I wouldn't worry about it unless it's rubbing on something. Tire sizes aren't exact.
It should be based on casing width. One manufacturer could wrap their tire tread far enough around that it would be included in a caliper measurement, while on another tire it might be a narrower band and not therefore falling in the measured width. However, they would have identical (all else equal) amounts of air volume.