Centre of mass

When centre of mass isn't defined right, you'll notice it when your car is spinning fast in the air. Let's study this with all three dimensions. (x , y & z)

X:
Well, you shouldn't need to do anything for this, unless the car is an odd one, or someone who sits in the car is very fat. Usually this should be zero.

Y:
If you raise this value too high, your car will flip over in a turn. (Just like a hammer, because the centre of mass is very high)
But if it's too low, your car won't "lean" in turns, which might mean a better handling. And there is a nice effect: you can drive up a steep slope, without flipping over!
(If the car's wheels have enough grip) Here are some values:

0.03 = Very low. You can drive up a steep slope without flipping over. For fun.
0.07 = Race car's centre of mass, which don't have very much ride height.
0.12 = Normal centre of mass, for common cars. (Normal ride height, unlike in race cars)
0.25 = For trucks and buses.
0.75 = The car flips over very easily, when rammed from sides.

Z:
Too far back, and the car understeers. You might crash into walls in sharp curves. The car's front will raise up during fast acceleration.
Too far forward, and the car oversteers. Then you might lose control easily, because the car reacts more quickly to steering.
If someone rams your rear wheels at high speed, you might lose your car's control easily.
These values are for normal 4-5 meter cars:

-0.10 = Very fast steering, not recommended. Could be hard to handle.
-0.05 = You could try this. But still a bit smaller value is better.
-0.02 = Could be an average value.
0.00 = Balanced steering.
0.03 = Understeering car
0.08 = Very understeering car. This value was used for Vlad.