On my commute to work I average 15 mph. There is about 800 feet of total climbing over 10-12 miles depending on the route that I take. I go as little as 4mph in places up the 1800 foot, 14% grade. My bike weighs about 35 loaded and I use 26x2 contental double fighter kevlar tires.

The max speed that I've hit on that bike was 48 drafting behind a bus and flying down the 14% grade. My bike is geared low, so I probably would have to be spinning about 150-155 rpm.

Doing about 150 rpm, I hit 51 on another 27x1 tire with a 52X13 this was going down an 10%, 2000 foot hill.

I have a Burley Rock and Roll tandem which my wife and I can pedal at a consistant 35 mph on a smooth, level roadway. It is nice to leasurely keep up with traffic and take up one lane of a 2 lane 35 mph road. We've toped that thing out at 48 mph going down an 8 % grade. Even with a drum brake it takes a good bit of time to stop.