What tire are you running? A larger aftermarket tire with higher rolling resistance will hurt fuel economy. If you are running taller tire, has the pinion factor been changed to correct the speedometer and odometer? This will make your mileage calculations be off. Do you have aftermarket steel bumpers? fuel tank in the bed? Brakes dragging? Worn front end components causing alignment issues? Air and fuel filter clean? All these things will have a negative effect on fuel economy. My $0.02.