I tried to put a car on my AmEx, having first verified with them that they'd OK the charge. (It was Platinum.) The dealer wouldn't take it. (I think they were more concerned with the AmEx percentage than with the possibility that I might return the car, since it was a high-demand model I had waited four months for that they could resell instantly.) I pointed out, politely, that in their AmEx contract they agreed to take it as payment for ANY transaction. They offered me a free LoJack (which I had ordered anyhow). I said OK.

So it can be worthwhile to try to use a card for a car even if you're pretty sure the dealer won't go along. Of course, you have to be prepared for them to call your bluff. It wouldn't look good if they find that your credit limit would be stretched by a second-hand Yugo.