I've made progress with the new vehicle system and it is shaping up nicely. The vehicle wheels consist of a slider joint with a spring (the strut) connected to a pivot, connected to the wheel by a hinge joint (the axle). If the wheel can be steered, an additional pivot is inserted between the strut and axle, with a motorized hinge to control steering. There were two big problems in addition to this that need to be solved in order to make a vehicle that is stable at high speeds.
First, the mass matrix of the tires needs to be spherical. The mass matrix is the distribution of mass across an object. A brick and a 2x4 piece of lumber probably have about the same mass, but have a different mass matrix. Therefore the brick should spin more easily than the lumber. If you don't make the mass matrix for the tires spherical you will get bad wobbling at high speeds, like this video shows:
When the mass matrix is fixed this problem goes away. The vehicle gets up to 90 MPH, and although there are other issues, there is no tire wobble.
The other issue that needs to be solved can be seen in the video above. At high speeds the tire collisions become inaccurate and the vehicle bounces a lot. We need to replace the default collision with a volume raycast coming from the top position the wheel can sit on the shock, going down to the extended position of the strut. This is the part I haven't done yet, but I know it can be done.
I think the new vehicle system will offer a lot of flexibility and possibilities for future features since it is mostly made with the standard physics features.