After looking at the configurations of the gearboxes in the Supercar kits 8880 and 8448 I decided to make my own out of the Lego pieces I own. Mine features 3 forward gears, and neutral. I did not have the right parts to make it a 4 or 5 speed with reverse so this was as good as I could make one. It shifts by pushing a gear selector forwards and backwards. The gears shift pretty smoothly though there are times that the gears don't mesh with each other correctly. I plan on using a variation of this gearbox in future large vehicles and I plan to make one with more speeds.
Excelent for a basic gear box, I especially like how shift lever rotates instead of slides like how most builders make their small transmissions (mine included). You should try to make it more compact.