To go faster you'll need a different ratio

To go faster you'll need a different ratio. To
ride downhill at 25 MPH with a 50 RPM cadence at the
pedals, you'll need a 5.6:1 gear ratio. A bike
with a lot of gears will give you a large number
of increments between a 1:1 gear ratio and a 6.5:1
gear ratio so that you can always pedal at 50 RPM,
no matter how fast you are actually going.

On a normal 27 speed mountain bike, six of the gear
ratios are so close to each other that you can't
notice any difference between them.

With actual use, bike riders tend to choose a front
sprocket suitable for the slope they are riding on
and stick with it, although the front sprocket can
be difficult to shift under heavy load. It's much
easier to shit between the gears on the rear.

If you are cranking up a hill, it's best to choose
the smallest sprocket on the front then shift
between the nine gears available on the rear. The
more speeds you have on the back sprocket, the
bigger advantage you'll have.

All in all, gears are very important to mountain
bikes as they dictate your overall speed. Without
gears you wouldn't be able to build speed nor would
you be able to pound pedals. The gears will move
the pedals and help you build up speed.

There are all types of gears available in mountain
bikes, all of which will help you build up a lot
of momentum if you use them the right way.