^^^ This. The tires alone will cause some issues with economy, especially if you’re trying to calculate MPG off of a calculated number. On top of that, with the lift, you will experience increased air resistance by exposing some of the less aerodynamic parts on the truck. From what I have read and experienced, the powerstrokes have the lowest fuel economy for DD. That being said, people like them because they do work. MPG is definitely best in an older cummins 2wd.