So luxurious! Very smooth ride, nice work with handling, and implementing the lights.
The one thing I'd do is give it a bit more roll. It corners pretty flat for a big ...saloon.
In my game I changed the masscenter to "0 0 1" and it became pleasantly animated during sharp turns.
My way to test and see if the masscenter is unreasonably high is to get in, orb, face straight down, and teleport, so it lands right on its front. If it tips forward onto its roof, the masscenter is too high up. I did this test, and it got back down on its wheels. So even if you land at a 90 degree angle, it'll still tend to stay upright.
I know it kind of stinks to ask you to change something after you've released it, but I think this is a nice, simple change.
Another thing that came to mind is that you may consider making the lights just a little bit yellow. In GTA V, one of my favorite things I noticed early on is that not every vehicle has the same color of lights. Modern cars' lights are a more bluish or pure white, and older cars' are yellowed. Not deteriorated-looking, but warmer. It's another small tweak that I think would improve the overall atmosphere of the vehicle.
Oh, and two more things, I guess. Please let me know if I'm overstepping my bounds, I want to be helpful, not picky, but:
http://puu.sh/icfus/c4802f8f6f.pngI don't know how a vehicle like this would be balanced, but I suspect that it would be a little more toward the front, rather than the back. A masscenter of "0 0.5 1" moves the slight imbalance to the other side:
http://puu.sh/icfUT/5e60d9f3ec.pngI would also consider making the vehicle rear wheel drive (and doubling enginetorque to compensate) by adding these to the function at the bottom of Saloon.cs:
%obj.setWheelPowered(0,0);
%obj.setWheelPowered(1,0);
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