Rear Axle Gear

I have had two ecoboosts and I recommend the max tow package. You get the 3.73 rear end, larger tow mirrors and better cooling. You also get an extra 400 lbs of payload with the max tow.
The main thing that kills fuel economy is larger, heavier tires and a heavier truck. My '11 Super Cab was over 1000 lbs lighter than my '13 fully loaded King Ranch and fuel economy was 2-3 mpg less with it.