since I have lived in the mountains of Virginia and Utah, I'm sure I've hit 60 more than once. One time I noticed I was going 60 on a downhill and I wasn't even in a full tuck. I wasn't really scared, but I decided that might not be the best idea. I used to go into a tuck on any significant downhill, but now I just do it if it's going to save me some work going up the next hill.