The guy who lives in Stilwell should already know by now that being in a car in a tornado is a very bad idea. It would not really be possible to tie down the car well enough to be certain it wouldn't be thrown around in the tornado. His house has a basement. When the tornado sirens go off, that's where they should go. If the house is actually hit by a tornado the car, in the garage, would probably be sucked up, along with anyone foolish enough to be inside, and deposited -- violently -- who knows where. If they are all in the lowest level, the basement, they may well survive.