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27 Inch
ISO 630 mm.
This is the size that was used on most older 10- and 12-speed road bikes made for the U.S. or British market from the 1960s through the early '80s.
Note on tire/rim compatibility: Traditional 630 mm (27 inch) rims were straight side design, but in the late '70s they evolved to a "hook edge" design which would permit the use of higher pressures.
These days, many 630 mm (27 inch) tires are marked "For hook edge rims only" (some companies use the term "crochet type" instead of "hook edge"...this is the result of poor translation. "Crochet" is French for "hook.")
Modern 630 mm (27 inch) tires will work on older straight-side rims, but they won't handle as much pressure as with hook edge rims.
Generally, the "rule of thumb" for traditional 630 mm (27 inch) tires is that they should be inflated to 70-75 psi. This shouldn't be a problem with any naarrow tire, despite whatever disclaimer the manufacturer puts on the sidewall. However, if you're restricted to this pressure range, you probably shouldn't be running tires narrower than 1 1/8", or preferably 1 1/4" wide, unless you're a very lightweight rider.
"27 inch tubulars "
Back in the '50s and '60s, it was not uncommon to refer to tubular tires as "27 inch" but this was actually an error. The largest size tubular fits 622 mm rims, which correspond to the 700c size. However, in that era, 622 mm clinchers were almost unknown in English-speaking countries. 27" (630 mm) was the closest readily available clincher size, and many cyclists got into the mistaken habit of calling tubulars "27 inch." This has caused a lot of confusion, especially when people try to find "27 inch tubular" replacement tires. Actually, no such size ever existed, it was just sloppy nomenclature.
Schwinn also caused some confusion because it often referred to "27 inch tubular rims." In this case, the adjective "tubular" refers to the way the rims were manufactured, not to anything unusual about the tires. Schwinn "tubular" rims were made from steel tubing that was rolled into the shape of a rim cross section. The 27-inch size takes normal 630 mm clinchers. (The confusion is compounded by the fact that many other older Schwinns did use non-standard tires, but that was only in 26 inch and smaller models. See also: Tire Sizing
