Angular momentum proportions are based on the total width, height and length of the car. (x , y , z)
X:
This value affects to steering and "barrel-rolling". If this value is very high, the car will roll sideways very slowly & smoothly, maybe like it had some helium in it. (Unlike in the first picture)
Y:
This affects to barrel-rolling too, if you put a high value here.
Z:
This affects mostly to steering. And if the value is too high here, and you drive down from a rooftop, the car won't lean forward when there's not anymore ground under the front wheels. (Second picture)
Here's an example:
Some values for different kinds of vehicles:
Kutter's Diablo: 0.40 , 0.22 , 0.77
Prop shafter (picture) 0.32 , 0.24 , 0.78
Pitbull's Big Dump 1.20 , 1.12 , 1.97
Limousine 0.44 , 0.32 , 1.41
Bus 0.60 , 0.64 , 2.60