Actually 650b is used when the tire width is less than 50 mm (randonneur and gravel bikes) and 27.5" when the width is over 50 mm (mountain bikes). Jan Heine an

Actually 650b is used when the tire width is less than 50 mm (randonneur and gravel bikes) and 27.5" when the width is over 50 mm (mountain bikes). Jan Heine and Kirk Pacenti reintroduced this size.