I used to be a 17-18 mph rider on my old Trek road bike. That was for 'pretty flat' centuries with around 1300 feet of climbing, and drafting other, stronger riders like a madman.

Now I have a lowracer and I typically do around 20-21 mph on century rides, no drafting required. That's rolling average; I rarely do centuries for elapsed time - rides are an excuse to eat! The last century I did for elapsed time, I did it in 4:38. As for max speeds on flat ground, I've been clocked at 39 mph. I should add that I still have an upright bike, and my speeds on that have actually gone down over the years; so I can't blame it all on being in better shape.