When centre of mass isn't defined correctly, you'll notice it when your car is spinning fast in the air. (every object spins around its centre off mass point, when airborne) Let's check 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 heavy. 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 its centre of mass point is very high above)
If it's too low, your car won't "lean" in turns, which might mean better handling. And there is a nice 'side-effect': you can drive up a steep slope, without flipping over sideways. (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 = For cars without 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, especially when rammed from sides.
Z: Too far back, and the car understeers. You might crash into walls in sharp curves. The car's front might also raise up during fast acceleration.
Too far forward, and the car might oversteer. And 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 more easily.
These values are for normal 4-5 meter long cars:
-0.10 = Probably 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 the average value.
0.00 = Balanced steering.
0.03 = Understeering car
0.08 = Very understeering car. This value was used for Vlad's vehicle.