I used to run 25" on my old XC bike, but I would get the ends caught in the tight trees. I switched to 23" on my current XC, and it's much better. My daily driver has 22" and is a handful to handle. No pun intended. It's very quick on the road. The narrowest I've ever seen was on a courier's bike: gold Answer Hyperlites cut down to 16". Thatz nutz!