The National Highway Traffic Safety Administration says the number one "clunker" being traded in right now under the Federal Government’s “Cash for Clunkers”, is the Ford Explorer four-wheel drive. The number one new-car purchase using a clunker’s rebate is the Toyota Corolla. So far, almost 185-thousand clunkers have been traded in through the "Cash for Clunkers" program.  Under the program rules, trade-in vehicles have to get 18 miles per gallon or less. They also have to be less than 25 years old on the day you trade it in.