Even a compact car (ones made within the past couple of decades anyway) has a curb weight of at least 2000 pounds. Since the front of the car isn't incredibly wide, it's unlikely you'll be making contact with more than 4 or 5 Zombie's at once. Assuming the average zombie weighs no more than 200 pounds, the car has at least a 2:1 mass advantage before you take into account the occupants of the car. As long as the car isn't underpowered I don't see a real problem plowing down a horde of zombies.
You're assuming that there's nothing behind those 4-5 zombies. Sure, the car has more mass than a few zombies, but not a huge crowd of them all bunched up against one another. Aside from that, a compact car would lack the wheels and suspension necessary to drive _over_ all those zombies, and thus would get stuck pretty quickly. That's why something like a tank or a monster truck would be preferable.