I've hit 65mph on a mountain bike with zero pedaling, it was a very very steep hill, but not very long. i did it once and lived to tell the story, i will not do it again, at least on that road

it was the most thrilling thing i ever done on a bike and yet the stupidest.
if anyone is considering going fast, just remember that the more speed you have, the harder it will be slow down! also, ride the hill a few times prior to going all out to check for safety hazards that may not appear the first ride or two...