There are just too many variables to do a really good apples to apples comparison between different trucks, locations, and drivers. You do tend to hear more from those having issues (low mpg) and you hear from a few getting really good mpg (I'm one of them). Likely most who are in between you don't hear as much from.

The ecoboost engines are known for getting really bad gas mileage if you drive it hard, but at the same token they can get really good mileage if you baby it a bit and keep the speed down. The same has always been true on any engine but I think it's just more pronounced on the ecoboosts due to how they make boost at low rpm.

I truly think if you gave me two nearly identical trucks, and I learned how to get the best mpg I could from both, that I would always get better with the ecoboost due my driving habits, patterns, location etc. That is why I went from a 6.2l to a 3.5 again (my 2nd one I've had).

That said, my brother drives a 14 3.5l and gets roughly half the mpg I do. Wish he would swap me for a week, i bet money I could get almost as good of mpg with his, and mine would tank to his mpg level. IMO it's mostly driver, with some help from tech advancements over the years, and him doing more in town and me doing more hwy miles that make up the difference. I doubt there is anything "wrong with his".

 

Another variable seldom mentioned is altitude. At altitude (even 5,000 MSL) air is less dense and so laee drag. My 2011 5.0 gets much better gas mileage in the mountains.

 

 

Well the o2 sensors keep the mixture at or near stoichiometric just like at sea level so no.
The NA engine does make less HP at altitude and is probably a little less efficient but the real benefit is lower aerodynamic drag.
Its why airliners fly > 18,000 MSL.

 

 

Actually airliners fly at altitude because of engine fuel consumption. The turbofans and jet engines are not efficient at low altitudes. That's why for lower altitudes it's better to use a turboprop.

 

I can tell you my F-150 Platinum Crew cab, 6.7 bed 4x4, with the 3.5 ecoboost, and max tow gets 19 mpg Hwy. Most the 18,000 miles on my truck are interstate. Truck has great power, and has a lot more features than the Denali did. But the 6.2L V8 in the Crew Cab, 6.5 bed 4x4 got better MPG than this truck. Driving same route at the same speeds the Denali got better mpg. Yet the window sticker on this truck says it should have got two better than the Denali did. I pretty much like everything better about the Ford Platinum than the Denali, except the fuel economy. Interior is roomier, better moon roof, better seats, keyless ignition, never take fob out of pocket. So I do like pretty much everything but mpg better about my F-150.

 

Gotta get my two cents in on this thought...

Sure, aerodynamic drag is reduced at high altitude, but so is throttle restriction. You're going to need power to keep going down the road, even with the reduced aero drag. The absolute manifold pressure isn't going to drop all that much. In the thin air, your throttle will have to be more open/less restrictive, reducing pumping loss.
that's not the whole reason, but it sure helps.

 

I just know that I got around 19 mpg actual at elevation and more like 16-17 near sea level. Since aerodynamic drag is one of the largest engine loads at speed, it makes sense.

 

I've noticed on the lie-o-meter that the relative mileage difference can be pretty significant when dealing with a strong headwind or tailwind. I've also noticed that it gets about 5 mpg better if you draft a semi, but that isn't a really good place to be in terms of safety.