There are two things that keep you from going faster: mechanical resistance, and wind resistance. Mechanical resistance increases linearly with speed, while wind resistance increases with the square of the speed. For an average bike, with a moderately upright riding posture, the mechanical and wind resistance are about the same, when you are travelling at 4 mph. This means that at 15mph, the wind resistance is 4 times as important as mechanical, and at 20 mph wind is 5 times more important. Very quickly, the only thing that really matters is the wind resistance - therefore, the key to increasing your speed is to present less wind resistance, which you do by moving your body lower, and more horizontal. You can do this on a mountain bike by hunching down and putting your chin on the handlebars, or on a road bike by moving your hands into the drops, the main thing that a road bike is doing for your speed is making it easier for you to maintain a more aero position. - If you really want to go fast, get a 'bent!