In a match that allows auto-aiming, cars are just deathtraps since all someone needs to do is hold and fire three bullets to kill you. Only use a car to get to the battle zone, not to plow through it--someone will notice and make fast work of you before you hit someone. They are horrible for cover as well since windows can be shot through. Also, a skilled player can still hit you by clipping your arm or leg because they always slightly show while in cover.
In a match that doesn't allow auto-aim, cars are very useful as it is far more difficult to hit the small part of your body that remains when you are using a car for cover. You can also drive through the battle zone with ease
When your car catches fire, avoid jumping out if you are being shot at. Keep driving as you have a good ten seconds before you must get out. That can allow you to drive to safety while bailing out (while driving) afterward, without sealing your own fate because you are a sitting duck when you exit any vehicle.