Originally Posted by palincss

There is no such thing as 27 x 1.25. It's 27 x 1 1/4. As you'll see on Sheldon's site, two tires with what should be identical widths, one expressed as a decimal and one as a fraction, will never be the same. What you need to pay attention to is the ERTRO designation, which is the bead seat diameter. 27" is 650, 700C is 622. Regardless of the width, those two tires will never fit on the same rim. It gets even hairier when you get to 26". There are quite a few 26" sizes: MTB 26" is ERTRO 559; 650B aka 26 x 1 1/2" is 584; 26 x 1 3/8" English 3-speed size is 590 and there are several other more obscure sizes called 26".

You may be able to convert your 27" bicycle to use 700C wheels by using longer reach brakes. At that point, the actual width of the tire -- what will actually fit -- should be your guide. In some cases, that may actually be wider than the original.