27" vs 700c
There is an 8 mm difference in rim diameter (700c is smaller). 700c x 35 will rollout about the same as 27 x 1.25. 700c tires are available in sizes from 23mm wide (some of which are closer to 21mm) which is about 3/4" up to 40mm wide - almost 1 1/2" in width. What will fit is a function of clearance between the stays, bridges and crowns. For reference my '83 T400 Cannondale was designed for 27 x 1.25 tire and fenders. I'm running 700c x 37 tire and fenders today for touring on gravel railtrails.
I disagree that 27" = steel rims. Plenty of quality alloy wheels were and are available in 27". Many quality bikes from the 70's and 80's were built using quality 27" alloy wheels (trek 520s, Fuji, Schwinn Paramounts, Cannondales, among numerous others).
There are a lot more choices of quality 700c tire sizes, but recently things have improved. You can get quality 27" tires such as Schwalbe Marathons, Panaracer Paselas, and Continental Gators. Even Performance Bike has their house brand available in 27 x1 and 27 x 1.25 with/without Kevlar belts for the budget minded.
In some cases you can swap out 700c wheels for 27" but in some cases you'll need different (long reach) brakes to make up the difference. If you're thinking about a switch, see if your brake pads can move down 4mm (about 1/4") to match the diameter of the 700c rims.
If you have a bike with steel wheels, well, they have got to go. Steel wheels barely provide adequate braking when dry and NONE when wet. If you need to replace wheels (and your brakes will work or you're willing to swap them also) then I would replace them with 700c
Last edited by dcullen; 08-13-08 at 07:26 PM.