Calculate your monthly car payment with down payment, trade-in, sales tax and fees, and see the total cost of the car including interest.
In most US states sales tax is charged on the price minus the trade-in, which makes trading in worth more than its value alone.
Longer terms (72–84 months) lower the payment but add interest, and you can end up owing more than the car is worth.
Many advisers suggest 60 months or less for a new car, and 36–48 for a used one, so you aren't paying for a car worth less than the loan.
It's possible, but you pay interest on the old car's debt too. Paying it down first is usually cheaper.