I'm running Panaracer Pasela tires, 27x1-1/8 on my commuter. They fit fine under the Zefal fenders. The clips that attach the fenders to the fender stays would likely interfere with anything bigger. I remember paying less than $20 each at a LBS. Nashbar sells 'em, so does aebike. The Paselas seem to be long wearing. Probably over a 1000 miles so far so far, and I have no plans on replacing them anytime soon. In fact, I can still see the seam from the mold down the centerline of the front tire. Walter.