Quick question: Do you have a 3D halographic display that lets you view a game from all angles? No? I didn't think so. The game doesn't draw the entire car, it draws the section that you are looking at. If you're looking at a car from one side, the other side of the car has no impact on performance since it doesn't exist. Also, you have to factor in that the further away a car is, the less polygon is actually has since the polygon count can be dynamically reduced.
Good news Everyone!
I've invented a device which makes you read this in your head, in my voice!