Cars are very useful as they provide quick means of transportation, destroys almost any type of zombies, and have unlimited fuel. The fuel that you get in the game is for making explosives so it doesn't affect the cars at all.
They have limited use however, as they take damage from bumping against anything and hitting enemies. Over time when a car takes too much damage, it blows up and gets destroyed permanently. To counteract this, you need to create a workshop in your home base and leave the car parked in your home and it will be repaired overnight.
As weapons of choice, they are extremely effective against Hordes and Infestations as they leave you very safe and take out so much more enemies in return. Simply run them over and unleash unliving hell on the undead! Downsides though are the facts that they are very noisy which draw unwanted attention and does not damage the Big Úns or Juggernauts which are the baddest enemies in-game. You can still ram an exploding car into one to deal very high damage to the big guys.
Press RT to accelerate and LT to drive backwards. If zombies jump and grab onto your ride, press x repeatedly to hit them with the door. Do this quickly as they can pull you out of your vehicle if don't act soon. Also, never jump out of a speeding car, press B first to slowly brake before going out safely.