With most bikes you can go down to 28's on a hybrid with no problem. It makes an instant difference. I felt a little nervous at first - you can't just plow through gravel or over lawns with utter abandon, though basically I could do most rougher terrain (gravel or grass, not real off-road) but just had to be careful. Tires are a whole lot cheaper than wheels :-) And you can ask at your LBS... I have 25's on my 7500 FX (hybrid, but more like a road bike) for going fast.