I would not want any tire laying loose in the back of my car, in the event of a sudden stop I would not want to be struck in the back of the head with a wheel.
Having survived (hopefully) a front ender and with my face planted in an air bag I would not want a wheel striking me from behind. The seat backs are not meant to stop a flying wheel. Even a roll-over would put you at risk. It would be ironic to survive a serious crash only to be killed by your own wheel.
The wheel would have to be tied down very well with a stud and not bungee chords or rope.