I have a 99 freighliner century. C-10 3:45 rears with 22.5, 10 speed auto shift. Drop axle with liftgate. Run around 62 mph. Have a seperate fuel tank for the apu. Avg for last month was 11.18 mpg with a avg weight of 2800 lbs. So far this month its 10.89 with avg weight of 5156, but this month...