I'm not sure - I drive a stick...maybe an automatic driver can chime in? You should be able to tell by the engine RPM's at 65 mph, though.

Didn't I see one of your posts that said you were smelling gas fumes? Maybe that is your problem - you could be leaking gas and that is killing your mileage...although you would have to be leaking ALOT of gas (several gallons worth) to drop you mileage that much.