I have a 3 spd with granny gear, running LT265/75r/16 with 4.56 gears, 350 TBI. Any ideas how to improve my speed??? I tow a 30ft 5th wheel from job to job but, still empty i get same speed. I only move from town to town maybe twice a year. Empty i get 13-14 mpgs at the ol'double nickle so i...