This is quite important, because it has a significant effect on the handling of the car. These values equal to the center points of each wheel.
X: If this is too narrow, your car will flip over in turns a bit more easily. If it's too wide, the game might crash and say: "Wheels outside all shapes"
That means that they are outside the bounding shapes. It's an annoying message, and it makes it difficult to edit cars like auto-scum gang members have.
So you have to make the bounding shapes wider, or put a smaller value the wheel's position.
The skidmarks seem to appear a little too much to center if you put the exact center point, so I usually add an extra 0.02 here.
Then 0.15 will become 0.17 , and -0.15 will become -0.17 , for example.
0.16 = Normal car's wheels positions.
0.26 = A bus.
0.37 = Pitbull's Big Dump. (The skidmarks appear in the center of wheel, but are too small...)
Y: I haven't really figured out what kind of effect this has, or if it has any kind of effect. I remember being told of something, but can't remember it anymore. :-) 0.07 is the default value, though.
Z: Many of the cars are modeled so, that the Z-value of the front wheels is different from the Z-value of rear wheels.
In other words: the Z-value is 0 at the center of the body, not at the center of the four wheels.
But in that case I always move the model so, that every wheel has the same Z-value. (Except that front wheels have the minus in front of them, of course)
That's how I get quite balanced steering, and the centre of mass will also become more significant. For example, if the front wheels are at -0.15 and rear wheels at 0.17, I move the car by -0.01 , and both wheels will have (-/+) 0.16 as Z-value.