My rear rim came with a 28mm tire. I put on a 38mm, it was tight, had to grind of a bit of frame and brake mount. Replaced it with another, absolutely no go, inflated tire totally locked to the frame, no clearance, did a bit more grinding, no way. Much different tire. Went down to a 35mm, HUGE amount of clearance. A 38mm of the same "profile" as the 35 would fit no problem. The "profile", not sure if there is a better word, exact height and width, is what will vary. If you go oversize, make sure it is a wire-beaded tire. Tried one of the other kind on same rim, did not end well at all. Funny story.