I have a 2008 F350 6.4l dually, I've upgraded my Intake, EGR/dpf delete with a 5" down pipe back exhaust. Also put in a new cold side intercooler pipe. I have the SCT competition tuner that came with 5 custom tunes. I got 16 mpg highway/city combined after I loaded my tune on the first tank. I've filled up twice since then and now I'm back to 10 mpg on the performance tune. I just had an oil change and replaced the high fuel pressure pump and sensor in the last 2000 miles. No codes so I'm not sure where to start on troubleshooting. If anyone knows what I can check into please let me know. I have 116000 miles on my truck, no lift or oversized tires. I've yet to try the towing tunes maybe that will help with mileage??