Originally Posted by
Carbonfiberboy
700c is actually 622mm, which is a measurement. A large difference between 27" and 622 rims is that 27" rims are straight-sided and 622 are hooked rims. Hooked rims hold the tire on better. Old tires can blow off 27" rims - had it happen. There is definitely a functional difference.
The biggest difference is that tire technology, not engines, is the major advance in automobiles, and it's the same in bikes. You can get more advanced tire technology in 622 (700c) than you can in 27". Whether you're into wide comfort tires or narrow racing tires, you can get better tires in 700c than in 27", in terms of comfort, handling, grip, and rolling resistance. Of course we can argue all day about what "better" means, but just look at the variety available in each of those types.
Not quite. If you measure across a 700c rim bead seats, you will get 622 mm. If you do the same for a 27 inch rim, you will get 630 mm. Both 700 and 27 inch are approximate, really just labels. That four millimeter difference in radius is why 700 and 27 are not interchangeable. The other numbers, 23 mm, 25 mm, 28 mm, 1 1/4 " et cetera are the width of the tire when inflated. The mounted and pressurized diameter of the overall wheel is what it is, not exactly predictable by tables or equations. The width does provide an indication of tire air volume.
Old 27 inch rims did not have hooked beads, and can only be safe with something like 80 psi or less. Some modern 27's have hooked beads, and the Gatorskins and Paselas can be used on them all the way up to their max inflation levels, around 120 psi. If you do not have hooked beads, you should get the widest Gators you can for your rim, so that the added air volume can make up for using a lower air pressure.