Have a 97-02 series f 150. wanting to tow a utility trailer with approx. 1100 pounds including the trailer. Should I leave transmission in OD 1500 rpms at 55 or 2200-2300 in 3rd? Not very hilly area and the vehicle has around 115000 miles. 4.6 engine no towing package. This would be a highway trip. What is best for the engine and transmission?