It depends on what you are doing and what you want out of your ride. If I'm racing I'm pedaling up to the 45-50mph range but there are diminishing returns the faster you go. If I'm riding with friends and want to put the hurt on them I will pedal on descents. Mainly because they are skinny little climber boys and they don't have the power that I have so it's fun to return the pain they give me on climbs. I would disagree with Mr. Friel a bit. If I'm racing, there is no way I'd be coasting at >50kph, that number would be closer to 64kph. As an example, there is a race I do that has a long (25mile) descent of 1-3% grade that many racers coast on at about 30-35mph, I put my bike in it's 60/11 gear and can easily hold 40-45mph the whole way and I always pass a boatload of racers through that section, some of them like they are standing still. One note, I run big gears, that would not make as much sense if you are running lower gears like a compact crank.