I find that when I hit a big downhill, I change into my large front chain ring (I have 3) and the hardest rear ring, and then build up some mighty speed (of course, lots of times I coast). When riding with my dh, I pedal in the higher gear just so he doesn't fly past me then lose me on the next uphill. When riding alone most of the time I just coast, but it's fun to get that speed up there- feels so out of control crazy and yet so exciting...