Joined: Nov 2006
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From: NJ, NYC, LI
Bikes: 1940s Fothergill, 1959 Allegro Special, 1963? Claud Butler Olympic Sprint, Lambert 'Clubman', 1974 Fuji "the Ace", 1976 Holdsworth 650b conversion rando bike, 1983 Trek 720 tourer, 1984 Counterpoint Opus II, 1993 Basso Gap, 2010 Downtube 8h, and...
Incidentally, a nominal tire size like 26 x 1 3/8" refers to a wheel that measures 26 inches including the tire, with a tire 1 3/8" (1.325") wide. So when considering the diameter, you have to include the tire with twice.
So if you do the math,
26 x 1.75 gives 22.5" or 571.5 mm (compare rim size ISO 571)
26 x 1.5 gives 23" or 584 mm (compare ISO 584)
26 x 1.325 gives 23.35" or 593 mm (compare ISO 590)
26 x 1.25 gives 23.5" or 597 mm (compare ISO 597)
You would get the same wheel diameter by putting a 19 mm tire on a 700c rim:
26 x 0.75 gives 24.5" or 622 mm (compare ISO 622)
Obviously it's not perfect, but you get the idea.