Originally Posted by
hamster
Just run the numbers through a calculator:
http://www.kreuzotter.de/english/espeed.htm
For a 6'3"/200 lb cyclist in the drops coasting down 8% grade at 5000' above sea level, it gives the speed of 44.2 mph. (I think that the calculator assumes an unreasonably low aerodynamic drag in the drops, but that's not a big deal.)
For an "average" 5'8"/150 lb cyclist going down 8% grade at 1000', the sustained speed is only 38.1 mph.
Calculators make assumptions that won't be true most of the time. 40 is really an easy speed to hit as others have mentioned -- I'll do that with one hand on the bars sometimes.
As far as the OP getting passed when he's pedaling and they're not, it's because aero rather than pedaling tends to be more relevant at higher speeds -- at least those achieved due to gravity. It can happen though. 2 weeks ago I was descending a two lane road, caught a car going 45, got bored (plus I knew I'd take a series of curves coming up faster than it could), and used the draft of the car plus a bunch of pedal power to slingshot myself around it. But except for the boost, it was still all about aero. Handling skills are also relevant to top speed when going down hills.