I just purchased a 99 f-250 4 door short bed 4x4 7.3 turbo siesel. It has 285/r75 tires a super chip set at safe tow and 3" exhaust. I am only getting 12.8 mpg not towing anything. I just put a new k/N air filter on. any tips on increasing mpr? I stay under2000rpm and don't excellarate hard the truck has not been used for tow yet. It was owned by someone in the airforce who was sent overseas so his wife has been driving it for family transportation only has 75000 miles