just put on air intake and exhaust and flash paq. hope to get better fuel mileage towing than i did. last weekend truck all stock got 7.3 miles per gal. towing 15,500 lbs. with new upgraded trk got 9.0 miles per gal. towing the same. tune: tow. is this normal with that kind of weight or should it have been better. 04 duramax gear ratio 3:77 any help would be great.