Its just a worktruck
Tuners only add mpgs if you drive like a grandma. I got my best mpgs with stock tuning around 70 mph. The MPG display on the dash is wrong. Add a tuner and the numbers get even more skewed. All aspects of a duramax are electronically controlled, even the torque converter.
There is no such thing as lean or rich on a diesel. They are throttled off of fuel, not air.I’m sure I will get flamed for this but here goes, IMO if all you are looking for MPG then the best item for that is your right foot. IMO any programming to get better MPG at a steady throttle is only doing so by running the engine to lean which is not good for the motor.