This should be easy: it defines the softness of the car. The smaller the value, the tougher the car. (Less softness = more toughness) If you want to have fun, put 0.0 for indestructible car. But remember, if the car were an opponent, you couldn't waste it. I'll just tell some example values now:
0.00 = Rock. Solid granite car. Invulnerable.
0.05 = Very hard to damage. It was actually used for Pitbull, although that truck wasn't so hard to waste because its mass often made it waste itself.
0.13 = Still very tough. This value was used for Abba Cab. (The red truck)
0.40 = A bit tougher than average. For example in Hick Pickup.
0.50 = An average car. This was used for The Red Vet.
0.70 = This was used very often for the original C2 cars: Eagle 3, Porker 2, Prop Shafter and Wide Boy.
1.00 = Soft. Lady Bug, for example.
1.50 = Very soft. Bugga had this value. (The pink off-road car)