I posted back on here a couple months back about my 2001 Cummins 4x4 automatic. It has factory wheels/tire sizes and is almost 100% bone stock. 102,598 miles. My last posting was about how the truck was getting 12 mpg or less. Thanks to ya'lls help I first discovered the fuel filter was completely plugged and had basically 0 PSI at idle. Got that fixed and saw gas mileage increase by about 1 mpg, but only for one tank. Gas mileage got down into the 10 range so I took it in and they replaced the dealer installed in-tank fuel lift pump that was replaced at 60k miles. I had them install a FASS factory replacement lift pump - not one of the high performance models. Right away I got 14.6 mpg in the city on the first tank. Then I filled up again and I was back at 12.1. WHAT THE HELL IS GOING ON. :) I did use some diesel Kleen right before the fuel lift pump was installed. Where should I look next to get my MPG's up? Thanks guys
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