Originally Posted by
jimmuller
I'm posting this to C&V instead of Bike Mechanics because it is more a reflection of how things were a few decades ago.
Today I replaced some old tires. The one on the front was a Specialized, marked 1-1/8", 110psi. It was noticeably narrower than the Pasela TG 1-1/8" I had put on the slightly narrower rear wheel. Okay, so that just shows that tires can be different even with the same markings.
But then I noticed that embossed on the sidewall was 23-630. Huh? No, it was not a 28 smudged out to form 23. I looked again. It was definitely 23. So what is it supposed to be, I asked myself? Wider than an inch or narrower? When inflated the sidewalls measured about 27mm, what I would have expected.
Curious.
23-630 would be the ISO tire size, the inner width of the rim is 23mm and the bead seat diameter of the rim is 630mm. ISO tire sizes are based on the size of the rim the tire fits, not the physical size of the tire, which would be about right for a 27" x 1¼" inch tire. Newer tires should have both marked on them, which the tires you have probably are.
Sheldon Brown says more
here