Truck is 06 2500 4x4 QCSB 189,400 miles. Ranch bumpers front and rear, leveling kit, 33x12.50x17 (tires actually measure 31.5"). I changed the fuel filter, strait piped the exhaust, and added a diablo predator on performance/economy. I tried cleaning my k&n filter and gained 1/2 mpg max so I tried a new fram and its the same mpg. I drive 20 miles hwy and 20 miles city every day and hand calc. 15-(16 max) mpgs driving easy trying to increase mpg. I have recalibrated the tire size to 31.5 with the predator so it should be correct. What are the problem possibilities? Ive been told I should be getting 15-16 city and 18-20 hwy. Please help and thanks in advance.
Zach

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3.73 gears auto tranny hwy driving always under 2k rpm