This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
If your car object had a toString() method that might work. But since it doesn't you'll need to specify something a bit more precise like car.getId() or something that can go into the value parameter of the option tag.