Best bet is to use sand bags. One more thing: tie a good rope or steel cable around the bags and anchor it to the body. If your car is doing 60mph and hits something, those cinder blocks will become 60mph projectiles. They will LITERALLY take your head off if they connect! The rope trick should act as a rip cord and you only get sandblasted. Saw a series of pictures once where a fellow with a 4x4 was trying to unstick a friend. Hook came loose from friends truck. Sailed through back window of Blazer and right through the drivers' head-rest, then windshield. Lucky for driver, his head was out of normal position as he was watching friends truck as he pulled. Not sure how strong the fuel tank behind the seat is in our cars. Probably would hold, but who knows. Wagon would be a lot worse.