Originally Posted by nemonis
You are partially correct. A 27" wheel does have a bead seat diameter (BSD) of 630mm. A 700c wheel has a BSD of 622 mm, so a 27" wheel is actually 8mm larger in diameter than a 700c. 26" mountain bike wheels have a BSD of 559mm, so they are 63mm smaller than a 700c (almost 2.5"). The reason for these oddities is that wheel sizes were the outside diameter WITH A TIRE MOUNTED. Only accurate for one tire size, and can be confusing as anything, but the way it was done. It is often easier to just use the BSD measurement.
God bless!
Wayne J.
That so called BSD is called ERTO and ISO within the industry. Avoid talking about 26" wheels, as there are 4 different ones and 6 different 24" and 3 different 20". Read the numbers in parthenosis next to the tire size on the tire to get the ERTO of your current tire before buying a new one.