Wow, did not think my comment would start a storm. It is simply not true that a 42-11 will "spin out" - that is, be unable to be pedaled faster - at 30 mph. Even for a non-racer 120 rpm is not outlandish for a short time. Saying that one does not want to spin faster than 100rpm is not the same thing.

As for pedaling downhill, it has two functions only - getting maximum exercise out of a ride and keeping the legs warm. If one is trying for maximum speed over a given distance (up and down course) it's wasteful to pedal rather than to tuck and coast, because wind resistance at speed makes the incremental cost of a couple miles per hour too high.