Dynamix' Tim Gift updated his .plan with an update on what he's been working on, and he gives a great deal of detail on the status of vehicles in Starsiege: TRIBES 2. Here's an excerpt, which sounds remarkably like something a used car salesman might say:

At this point you can drive around in the scout vehicle. It has four wheel drive and independent suspension. It handles pretty well, though at high speeds it tends to under-steer. Have to be a little careful going fast on the uneven ground or you can end up upside down. Vehicles are going to be pretty fun :)

All the engine, suspension and tire parameters are specified in the datablock. The number of wheels, which ones steer, and suspension travel is all extracted from the shape file. It should be fairly easy for mod authors to build and customize new vehicles.