meb

In the old French system, 700 referred to the outside diameter in mm and a/b/c/d referred to the tire width.

The 40mm c-width mapped a 700mm outside diameter with a 622mm bead diameter.

Eventually 700c came to dominate and all 622 bead diameters came to be referred to as 700c independent of width. The road tires are generally narrower so it results in a smaller diameter for 700c than 27 inch, even though 700mm is larger than 27 inches. The 28 name came to be applied to 700c to distinguish from 27 inch road tires particularly since the 28 inch 635mm bead diameter roadster tires (well suited for cobblestone) were disappearing and 26 would have confused with the common 26 inch road tires (generally in the 584-600mm bead diameters-brand specific). This in spite of the 700c road tires actually being smaller than 27 inch tires.



29ers- at 622x50 give a 29 inch outside diameter so 29 is actually reflective of their outside diameter.



I’ve generally been hearing the 650b mountain bike tires referred to as 27.5 (In the old French system b was a fairly moderately narrow general tire but wide road tire at 32mm wide tire-using the bead from the old narrow 650b on a wide tire gives a larger diameter) as a compromise between 26 and 29 rather than reflective of their diameter. The 650b road tire size is still common in France only.