A while back I was making some large LEGO vehicle of sorts, and I was faced with the choice of gearing down my motor or dedicating a whole extra motor to controlling a gearbox.
I chose to do neither, and build an automatic one. The ones I found where to big and relied on friction and differentials. After a few email exchanges and iterations, I arrived at this compact 2-gear automatic gearbox.
The gear has 2 sides, one with a 1:1 ratio, the other with a 12:20 slowdown.
When no torque is applied, the front lever is pressed onto the 1:1 chain by a rubber band, causing it to turn faster than the 12:20 side, and thus pushing the rear lever up.
When torque is applied, the front lever is pushed up by the force, and starts to slip, making the 12:20 chain turn faster, causing the rear lever to fall back into position.