Check out this cool demo for the Prius' main gear.

http://eahart.com/prius/psd/

There is a simulation at the bottom of the page that let's you vary the input parameters and watch the gearing adjust.

The PSD is a planetary gear set that removes the need for a traditional stepped gearbox and transmission components, and also the familiar rev-lurch-rev-lurch of acceleration in an ordinary gas powered car. It acts as a continuously variable transmission (CVT) but with a fixed gear ratio.

Cool stuff.  They need to get their car designers to work on the car though because it is the ugliest thing on the road...well maybe that Pontiac Aztec thing was uglier, but it's close.