Back in the summer, my 2014 F150 Screw with 3.5L ecoboost got 17.9mpg average for all the spring/summer. Some less but towards the end, I was getting it figured out how to drive it and get the best mileage. With winter set in frigid cold, start-ups to warm up, etc. the mileage is low 16. I'd sell it tomorrow if it dips down to the 13-15s you're talking about.
For reference, my daily commute has virtually zero flat ground for more than a mile or so at a time. Constant rolling hills pretty much. And I'm pretty even 50/50 stop n go versus interstate (70mph). If I'm not in a hurry and traffic is light on the interstate, I'll sometimes run about 65mph instead of the normal 72mph and it'll do a lot better on fuel.
Also, my truck is 100% stock with stock size tires on 18" FX4 wheels. I really want to go with aftermarket 20" wheels and a slightly more aggressive AT tire (something like the Nitto Terra Grapplers or Toyos etc instead of the road tread bridgestones it has on it now) but I'm having trouble finding lightweight 20" wheels and tires.