2017 F350

taken wrote:
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.

If you lower your road speed, you can get the same or better mileage with lower ratio (higher number) gears, particularly when loaded/towing very heavy in mountainous terrain.

Low ratio (high number) gears make for worse mileage, only because they increase engine rpms, for any given road speed. Lower the road speed until the engine is spinning the same rpms, and you get the same or better mileage, since the engine is working less with the lower ratio gears.