well, 1-1/4 is about 32 mm. 1-1/8 is about 28.5 mm... that should give you some idea of the range of the tube for your 27" wheel. Most are usually marked with both systems, but I've seen tubes at walmart and hardware stores that were only labeled for 27" sizes.


Be sure to match your valve type! It'll be difficult to find a really thin road tube with a schraeder valve, if that's what you need... you might consider one of those hole shims that will fit a presta tube into a schraeder hole.

FWIW, I've used really thin presta tubes (23 mm) in a 32 mm tire before without any trouble, but I wouldn't put a 32 mm tube in a 23 mm tire.