I was chatting with my boss (a competition-grade triathlete with several bikes who is much more hardcore than I am) about my average commute speed now that I've got a speedometer to measure it with. And on a flat segment of pavement, I can usually maintain 19-22 mph, which is allegedly impressive for a non-athlete on a mountain-ish bike. We decided it's largely the tires, since that's the main difference between my bike and a mountain bike. The way he said it was "I bet you are getting a couple of extra miles per hour because of the tires," which then required me to follow that with "Yeah, and I think I can get a few more with an exhaust tip".
Copyright 2007, Ken Wronkiewicz