Originally Posted by cyccommute

Although I detest responding to a very ancient thread, you’ve got things a bit mixed up. 27” wheels have a 630mm bead diameter. With a “proper” sized tire, the overall tire diameter is 27”. Given that a 630mm bead set diameter is 24.8”, that means the tire should be 1.1” in diameter to make the wheel/tire system 27”. 29” is a designation that mountain bike guys came up with for a 700C (622mm) rim with a 2.25” tire on it.



The French system maintained the same outside diameter of the tire but changed the bead diameter (and rim diameter) to do so. A 700A used a 642mm bead diameter with a 29mm tire diameter to achieve 700mm outside diameter. A 700B used a 635mm bead diameter and a 32.5mm tire to achieve a 700mm outside diameter. A 700C used a 622mm bead diameter and a 39mm tire while a 700D used a 583mm bead diameter and a 58.5mm tire. It was a throughly confusing system and, thankfully, only the 700C remains. Note that in this system a 700C is almost the widest tire that was available under the French system.

The 650 system did the same but with a correspondingly smaller overall diameter. The 650B and 650C still exist as a rim diameter but not the tire system.



Yes, ETRTO is much better, although they get 37mm wide tire wrong since both 35 and 37 can be marked as “35mm” or “37mm”. It has to do with the height and width measurement. However, if you don’t know the old system, heaven help you with trying to find tires for some wheel. And heaven help you if someone comes looking for a 27” tire. It could be an old 27” or it could be the misnamed 27.5.