This is a working Lego Automatic Transmission.
The two main components are I technic diffrentle, and a spinning gear rack.
The way this works is the differential measures how fast the wheels spin, and when they don't spin fast enough
The differential will start to turn a little piece that unlocks the spinning gear rack.
When the rack is locked in place the motor is moving the gears in the rack faster there for there is more speed but less strength, and when it is unlocked the rack spins freely there for less speed but more strength.
And then when the whole rack spins the wheels fast enough the differential will stop there for locking up the gear rack.

If you have any comments or questions you can contact me at dillardtrd@hcsmail.com

There is also a person that I would like to mention that has made another Lego Automatic Transmission.
That I got the idea from and without knowing what he did I probably wouldn't of known how to do this.

Anyway you can visit web site here http://www.last-outpost.com/~malakai/lego/