Yeah I guess I'm just going to have to keep two sets of tires - the ones I got and some fast rolling XC tires. I didn't really want to change them all the time though. I definitely took a big speed hit on hard flat surfaces, so I'd like to swap out if I want to keep up with my XC racing buds. One thing I noticed was that the knobby slapping sound goes away when I'm turning. It's only around when I'm straight up and down. I also noticed that my center knobs are spaced out pretty far apart. I'm wondering if the speed hit is the result of this row of knobs each individually slapping the ground. I saw that one of the Maxxis tires, I think the high roller, has cut edges to eliminate this. I'm probably crazy, but I wonder if I could do a home grown cutting job on this center line of knobs and get a little more speed.