First of all I want to credit this guy:

http://www.last-outpost.com/~malakai/lego/

His 3-speed automatic gearbox is fantastic.
The use of 2 coupled diffs gives 3 different speeds.

All I did was change the package and add a little something of my own.

Now how it works:


The 2 differentials are the most important components.

AS can be seen in SAG3_int001.png. Green is the low-gear, pink the high-gear.
( the middle pink 16z gear is a clutch gear )

Blue is the input clockwise. Red the output clockwise.

Stopping green from rotating gives you high-speed.
Stopping pink from rotating gives you low-speed.
Stopping nothing from rotating give you something in between.

The yellow components in SAG3_int001.png are for changing speed. The selector.
In the front there's a worm-gear driving a clutch-gear.
In the back a standard differential.

The gearbox will always try to select high-gear. Low-gear is stopped.
But if the force coming from the rear differential is greater
than the clutch-gear can handle, the gearbox will first release the
low-gear. If this is not enough to transmit power, the selector will
continue to rotate until the high-gears are stopped.

I think this about sums it up. I guess you have to build it to see for yourself.