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Could be a problem that the DPF is restricting on the exhaust system but it should throw a code. I am running a 09 dually long box CC 4X4 with 3.73 gears and have averaged exactly 10.5 mpg hand calculated over 6700 miles pulling the 09 3400RL (my lie o meter will always indicate about 20% above hand calculated numbers). My speed is 62 to 65 mph unless we are pushing a wicked head wind we'll drop to 60 mph. Empty we average 16.5 on highway at 67 to 70 mph which is 1900 to 2000 rpms. I keep a running log of mileages,fuel,cost,where, when, and oil changes inclusive of fuel filters.
I am running a DPF delete (straight exhaust) and a Spartan tuner tuned at 210 horsepower in addition to the 350. Exhaust temp runs at 680F pulling and no wind and 950F with a heavy head wind. Truck has 29,500 miles now with absolutely no power train problems. That DPF filter can create many headaches with restrictions, horsepower loss, high exhaust temps, plus diesel pushed into the oil system.
If that person is running 4.30 gears with the tow boss pkg. at 65 to 70 mph, it will burn fuel!!!
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