Ok, some experimentations later:
I managed to get a pretty well behaved top-down tank using the following approach:
- Body for chassis.
- Body for left track.
- Body for right track.
- Use two pins per track to keep it positioned next to the chassis.
- Use gear with disabled joint correction for angular friction on chassis. (against static body)
- Use pivot with disabled joint correction for linear friction on both tracks. (against static body)
- Propel tank by adding forces on the left and right tracks.
If you apply opposite forces on L/R tracks, it will turn in place.
If you apply the same forces, it will travel forwards or backwards.
You can probably do something similar for cars, but with body per wheel, instead of body per track.