1999 Ford F-150 exhaust system problems

Poor gas mileage; has averaged 15.4 miles per gallon, and ford advertises 16 city to 20 highway. Other f150 owners report poor gas mileage. Is this false advertising because I would never have purchased truck getting 15.4 miles per gallon when I travel mostly highway miles. I have a loud rattle noise under truck and the catalytic converters and muffler have all been replaced! Also, vibration via gas pedal from engine (?) noted between 45 and 55 MPH.

Dt: bought a used truck, and after the warranty expired had to replace the starter/the ball joints/he upper tie rods, and the control arm on both sides. Engine manifold split, and exhaust system rusted. Later, while driving tie rods came off. There was a slight drop at the entrance to the parking lot, and as vehicle went back up the drop, it slid and there was an impact. Got out of the car, and saw that the tie rod came off and gouged into the pavement.