Hate to tell you, but a taller gear isn't going to make you go faster.
You need to work on pedaling faster, instead. Figure out how to count how many times you push the pedals around in a minute; I'm guessing it's around 40-60 (it's called cadence, FWIW). Now practice shifting DOWN a gear, and pedaling 10 rpm faster. After a few weeks, once that seems normal, try down-shifting and pedaling another 10 rpm faster. Once you've worked your way up to a cadence of 70-80, you may find you're already going 2-3 mph faster than when you started.