For strategy's sake -- and not for altruism, or any other bullshit like that -- Cindy should have given up the cars. She had to have known it would create a sense of jealousy in the camp. And, if it weren't bad enough that she denied them all the opportunity to have a new car, she kept bragging about how much she loved it until they couldn't listen to anymore. If she would have given up the cars, she would have at least guaranteed herself a Final Four spot. Rafe already said there was no way he would have voted for her. I doubt Stephanie would have either. But she managed to isolate the majority of them with what she did.
Besides, not for nothing, but Mark Burnett has a tendency to reward people who give up their prizes for others. There was another season, I believe it was in the Amazon, where Matt gave up his reward to see his mom to allow everyone else the chance to see their parents. In the end, Probst gave him an even better reward by allowing his mom to stay at the camp overnight. I bet you, if Cindy would have given up her new car, she would have had a nicer one waiting for her when she got back from the Reward Challenge.