Originally Posted by cyccommute

It’s actually more confusing than that. The whole lettering designation of the French system is a left over. Wheels with a 650, 650A, 650B and 650C are 4 different sizes of wheels and tires. The point of the letter system was to have wheels the same “size” (outside diameter) with different wheels. A 650 rim (597mm ISO or “Schwinn” size) was meant to be used with narrow tires so that the outside diameter was 650mm. A 650A (590mm or “English 3 speed” size) used a slight wider tire with the same outside diameter. A 650B (584mm) used a 1 1/2 inch tire and a 650C (571) was meant for 2” balloon tires. All 4 were supposed to have the same outside diameter. The 700 system had a similar sizing system. The 700C is about the only thing that survived and it was meant for wide tires...kind of fitting that it’s used for the “29er”.

I’ve been pushing using the ISO size at my co-op classes because of the confusion, especially when it relates to the 27” (630mm) vs 27.5er (584mm) conundrum. Lots of people come in looking for 27.5” tires and want to grab 27” tires off the shelf.