Originally Posted by
HTupolev
ETRTO is a standards organization. People sometimes use "the ETRTO" of a wheel when referring to its bead seat diameter (BSD), because ETRTO specifies standard BSDs. Bead seat diameter is basically the diameter that the bead of the tire sits at.
622mm is the bead seat diameter of a 700c wheel. "29er" mountain wheels also use a 622mm bead seat diameter, which is why the 40mm 700c tires were able to fit on your wheels.
Similarly, 650b and 27.5" both refer to a bead seat diameter of 584mm.
"26er" MTB wheels have a bead seat diameter of 559mm.
650c wheels are 571mm.
The old 27" road size has a BSD of 630mm. This confuses some people since it means that 27" rims have a much larger diameter than 27.5" rims. The reason for the discrepancy is that the marketing names for the rim sizes are usually based (very roughly) on the expected diameter of the inflated tire, and people usually mount much fatter tires on 27.5" mountain wheels than on 27" road wheels.
There's a whole other world of 26" tires you haven't taken into account as well. I won't go into it other than to say that 26" fractional (26x 1 1/2") and 26" decimal (26x 1.50") are two different sized tires.
That said, we would be much better off if people referred to tires by the ETRTO or ISO sizes. In other words the bead diameter.