While a higher gear (bigger chainrings and/or smaller cassette) might help you exceed 30mph on a downhill, you'll find that you can do as well and sometimes better by developing an aerodynamic position and coasting. Why work when you don't have to.

An effective downhill position is pedals horizontal at 3 & 9 o'clock, knees in against the frame for stability, head and body low over the handle bars, and elbows in. If there's cornering involved the outer leg is extended and some weight transferred to the outer pedal. With effective downhill technique, you'll be able to coast past people who are pedaling (something that tends to annoy them to no end).