With its aluminum construction being much lighter than the current model, the 2015 F-150 should have a significant increase in fuel economy.
However if you're just comparing 5.0 to Ecoboost, you won't see much difference in fuel economy. I have 3 different friends and family with Ecoboost F-150s and none of them get more than 15mpg average, and no better while towing than you get now. The biggest advantage of the Ecoboost is power, not fuel economy. If you are very light of the pedal, the Ecoboost will get great mileage, but step on it at all, and those turbos are sucking alot of air, negating much benefit of a smaller displacement.
One of my family members traded his Ecoboost F-150 for a hemi Ram 2500, without that much difference in fuel economy for him.
