One of the hard things to explain to customers in the bike shop was wheel and tire sizing. A wheel that takes a 26x1-3/8" tire will not accept a 26x1,75" tire. Some 26x1-3/8" tires will fit on a Schwinn wheel, and some English roadsters, but not on any other wheel. And, a 27" wheel is not 27" in diameter.
And what the heck is a 29", anyway? The 29" tires fit on 700c rims (the same size as modern road bikes).
Tire sizing is not based on the size of the rim, as they are on motor vehicles. Tire size is the measurement from the ground to the top of the tire, along a vertical line. In other words, it is the outside rolling diameter of a fully inflated tire.
A 700c wheel is 622 mm in diameter. A 700x45c tire on on a 700c rim measures 28" in diameter, and that is how they are referred to in Europe. (Confused yet?)
A 700x2.35" tire mounted on a 700c rim measures 29" in diameter, and that is where the mtb tire size comes from.
There are a number of 26" wheel sizes (650b, 650c, mtb/bmx/cruiser which are all the same diameter, and plain-ol' 650 which is the old Schwinn S-7 size). All of these wheels, with the "standard" sized tire will have a nominal rolling diameter of 26", even though the rims are all a different size.
Large mountain bike tires exist for the 650b rim, and they produce a 27.5" diameter! Uuhhh...!
A couple of years ago, I bought a 1962 Hercules 3-speed, and went to buy tires for it. Thinking that it had the English roadster size wheel, based on the tire width, I bought the S-7 Schwinn tires. Of course, when I got home, I discovered that the wheels were actually the 26x1-3/8" rims.
Another few years working with bikes, and I might get this straight.
This past winter, Tony asked me if I would install some cool white tires on his Schwinn 3-speed. I told him I would, so he rode the bike over and brought his new tires. Of course, he had S-7 rims and 26x1-3/8" tires...
So, a trade was instigated, and I ended up with a set of white tires, in the correct size for my Hercules, and Tony rode home on my gumwall tires, which were the correct size for his bike.
Sing along, " The wheels on the bike go round and round, round and round, round and round..."