Originally Posted by
79pmooney
The "c" in 25c is millimeters. But millimeters kinda like the centimeters of an exercise ball. A "65cm" exercise ball is rarely gong to be inflated to 65cm (unless that owner is obsessive). Tires widths depend on inflation, age, rim width and manufacturer (and probably batch ).
What? No. "C" doesn't mean millimeters in any other context.
The "C" in tire sizes stems from the 700C wheelsize designation. Over time, it detached from the rim measurement and attached itself to the tire width measurement, to look like 700x23C. A 650B tire is similarly described as 650x42B, an English 3-speed tire would be 650x35A, etc. If it's confusing, blame the French.