>>1139678Welcome to the result of roughly 100 years of designs from at least 5 different nations, some of which changed systems of measurement at some point. Add a dash of late capitalism marketing wankery, (hurr durr we'll call it "27.5"), and it's a complete clusterfuck.
Here's what you need to know:
ISO tires sizes are described as xx-yyy, where xx is the tire width and yyy is the bead seat diameter (BSD) of the rim, both in mm. There are only three common BSD sizes today (outside of kids bikes, BMX bikes, recumbants, etc).
622mm BSD = The old French 700C size if you're a road biker; universally the wheel size sold on modern adult road bikes. "29er" is a relatively recent term made up by marketers to sell fat tires in this size to MTB riders ("They're bigger than 26" tires, see?"). Some road tires in this size are called 28" if you're German and/or like Schwoble tires. Why do some German companies designate their stuff in inch sizes? Nobody fucking knows.
559mm BSD = modern 26" mountain bike tires, although the industry keeps trying to kill this size in favor of "29ers" and "27.5".
584mm BSD = The old French 650B size, which is making a minor resurgence among folks who'd like to ride wider tires on their road-ish bikes. Also "27.5" if you're a mountain biker, because the marketers thought we need ANOTHER name, and that MTBers would be too dumb to figure out that it was midway between a 26" and a 29" tire unless it was spelled out for them.
The latter is extra unfortunate because prior to MTB marketing departments making up the "27.5" moniker, 27" referred to a rim with a 630mm BSD, which was an extremely common size for road bike wheels in the 70s and 80s.
Those are really the only three sizes you should care about if you're dealing with a modern (built in the last ~20 years or so) bicycle.
