I can not find it at the moment. But, it said something like..If you have a car that is paid for and the gas is around $3.75 your car will run you a little over $3k a year in gas. Now, if you trade in your car and have a car payment PLUS gas you will be spending $6 to $7k, due a car payment, higher insurance rates(since you have a payment), and gas.
I have a SUV and it is paid for, I get around 18mpg in town. I try to run errands when I am out and about.
I do not feel that trading the car...JMO.