It is easy to get confused by tire dimensions, especially tire diameters. For years Americans referred to their road wheels as 27". When you ask for a 27" replacement wheel for your bike, and if the shop owner interprets this in its strictest technical sense, he will give you a wheel that is slightly larger than the 700c, which is a very nonstandard wheel nowadays (27" wheels are almost nonexistent on new bikes). What you really wanted was a 700c wheel, most likely. But there is an even better, more precise designation for that, which is bead diameter.

26" wheels are commonly used on triathlon specific bikes, but here again there is a problem in nomenclature, as a mountain bike wheel is also referred to as 26", and the sizes of these two wheels differ drastically. More confusing yet, if you ask a European what wheels he has on his standard road race bike, he'll likely say, "28 inch." He is talking about the wheel North Americans commonly refer to as 700c.

Dimensions in inches, 24", 26", 27", and 28", don't really correspond to any real dimension on the bike. A 26" wheel will have an inflated tire diameter of about 24 1/4", and the rim will be about 22 7/8" in diameter. Likewise the other sizes will yield diameters smaller than their nomenclature would suggest. Better to use bead diameter as a measure, since this is printed on every tire, and you can't go wrong if you specify this number when talking to a mail order salesperson over the phone.

The bead diameter of a road 26" wheel, also known as a 650c wheel, is 571mm. If you have a bike built with so-called 26" wheels, or 650c wheels, the tires you need have 571mm bead diameters, and this is what you need to specify when ordering.

A mountain bike 26" wheel has a bead diameter of 559. A full size road race tire (i.e., a 700c tire) will have a bead diameter of 622mm. These bead diameters are also known as ETRTO numbers, and ordering this way will keep you out of trouble.