I tried to pay for my wifes new Navigator with my Centurion Card but they stated they did not take credit cards for new cars.

I have purcahsed 2 $50,000+ watches with my card with no problems.

Owning a business I can say that the main reason places would not take a card for large purchases would be the fee involved to the merchant. Cars don't have the high profit margins that other things like watches have.