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How do I diagnose my fuel economy problem

I have dual 100 gallon tanks on a 2009 kwt660. When the red light comes on at the gauge near empty I can only put in 120 gallons. The gauge will read full but 150 miles later a quarter tank will read depleted at the gauge. Also the Ecm says I'm getting 7 mpg but my math says 6 and if I divide the 200 gallon by each quarter and divide by my mpg then those two numbers don't match up. It will show 3mpg. I want to be able to go 600 to 700 miles on a half tank. I'm getting 300 miles. I just had a new dpf put on and an overhead done. It did not help. Could it be a fuel restriction problem, a sending unit issue, a gauge calibration problem, or do I need a new engine?

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Figure your fuel mileage when you fill up but only figure the amount of fuel you put in not the amount the tank holds. Always fill to the same spot, I fuel till the fuel starts popping back out of the neck and stop. This will be closer than any ECM numbers. Keep up with it in a little note book write down gallons, mileage, where you fueled......

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What kind of engine you have?

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2009 Cummins ISX 400 ST.

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johndimariangel will be eternally grateful.
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