2017 F350

It's got nothing to do with tuning. All the pickups are tuned the same. 3.31's are more shallow gears and by nature will get better mileage than 3.55's. Step up to a DRW 350 or a 450 with 4.30's and you'll get worse mileage yet. It's the nature of gearing. The steeper the gearset, the more torque multiplication you get to the ground at the cost of fuel consumption.