Originally Posted by Maddox

That's one way you can do it - keeping the wheelset you have now and not buying a new rear wheel.

Check what size is marked on the side of your tires. It will either say 622mm or 630mm somewhere on the side. 622 means 700c rims, 630 means 27" rims. If you want to save money and run a brake, go with what Mickey said. You could ride and save up for a new wheelset. Just don't do any massive skidding on it, or you may tear the threads and wind up being (literally) unstoppable.

So yeah, check wheel size and your budget. You can get thinner 27" tires, and 700c tires come in all sizes, down to the ridiculously tiny (19mm, etc) -- but I don't know that I'd recommend you go any smaller than a 23mm on an urban fixed gear. I'd just as soon get a 25mm or 28mm with reinforced tread and low rolling resistance. Speed of the wheel has little to do with tire diameter and more to do with the tire's tread.