Interesting topic. I don't have computers on all my bikes (and usually don't turn them on when I do), but sometimes I record my rides with Cyclemeter/iPhone so I went back through the history. I've only had the app for a year, so this is fairly recent data.

The fastest I've descended was on my mountain bike (with slick tires), 41 mph tucked on a downhill part of Highway 50 during a circumnavigation of Lake Tahoe last summer. We were riding in the traffic lanes, watching our helmet mirrors, when cars approached (typical car speed is 60 mph there) we'd move to the right, they'd pass, and we'd swing back into the center of the lane. I didn't want to ride 40 mph in the narrow shoulder with storm drains and debris, too dangerous.

As far as max routine speed under my own power on my daily commute, I usually sprint up the eastern approach of the bridge I take (Burnside Br.) and on a good day, no headwind, if I feel strong, am not carrying much in the messenger bag, time the lights and hit the start of the approach around 20 mph, I can crack 30 mph (max recorded so far 32 mph). That is going up a slight grade (guessing 1-2%?), and I blow up by the mid-point of the bridge, so maybe a 200 meter effort. Going home, two of my routes put me on major streets without bike lanes (NE Sandy, NE Broadway) where I try to ride as close to car speed as possible. That puts me around 30 mph (max recorded so far 31 mph), between stop lights. At that pace, I am going about as fast as the slowest cars, so I'm not holding up traffic too badly. But I'm grateful when traffic slows for lights, because I can't keep up that speed for more than a few blocks. If there is a headwind, I can't do it at all.

The max sustained speed that I can keep up for, say, a couple miles, is about 25 mph. Has to be no wind, and I'll be in the drops. I'm not "cruising", but am working hard.

On long rides, I can average about 17 mph if the route isn't hilly. On Seattle to Portland last year, I averaged 17.03 mph on day 1 to Centralia which is 99 miles (fresh legs, almost entirely flat), but only 15.99 mph on day 2 to Portland which is 104 miles (tired, rolling hills, long grind on highway 30). On the 50 mile training rides for STP I averaged 16.5 to 17.5 mph. Going around Lake Tahoe, the average was only 15.15 mph, that is 73 miles (on mountain bike, some pretty hilly parts at Emerald Bay and then on Highway 28). This excludes stops, using the app's automatic stop detection. On each of those rides, I recorded a max speed around 29-31 mph (except the Tahoe ride), but those were probably descents of rollers, not self-propelled on the flats, because I don't remember sprinting on those longish ride days.

Looks like to sprint 30 mph up a 1% grade I have to put out about 750 watts, to ride 30 mph for a few blocks on flat road needs about 600 watts, to sustain 25 mph that is about 325 watts. So that tells me the power I can output for a couple minutes is what a pro rider can output for an hour or more :-(.