Rigs of Rods has gone open source as well. I'm actually working on some differential improvements, and engine improvements in the way that the truck idles. I basically have simulated a torsen differential. If you are completely straight, and dump the clutch, both tires spin. It just gives more torque to the slower wheel, basically. Except, just like a real torsen differential, if one wheel is in the air, then it'll act like an open differential.