I would have agreed with you save for a recent discussion that I started in this thread. Tom said "I remember a good technical discussion with one of the engineers on the EcoBoost engine here on FTE a few years ago that talked about this. Low boost levels don't necessarily enrichen the mixture, so often it's more efficient to run under boost than to run in the vacuum at high RPMs."

That got me to thinking back to a college text on the Fundamentals of Internal Combustion Engines wherein the advantages of Brake Mean Effective Pressure were discussed. And the upshot is that a higher BMEP gives higher efficiency.

There are lots of ways to increase BMEP, and one is to increase compression ratio. So, all else being equal, the engine with the higher compression ratio should be more efficient. But, you shouldn't compare compression ratios between naturally-aspirated engines and turbo-charged engines since the effective compression ratio of a boosted engine can be much higher than the static compression. For instance, this site's calculator says that an engine with a 10:1 static compression but with 10 psi of boost, which an Ecoboost can do, has an effective compression ratio of 16.8:1 - assuming it is operating at sea level.

And, there's another flaw in your thinking - altitude, which comes with mountainous areas. The above-linked site says the same engine running at 10,000 feet would still have 14.8:1 effective compression ratio. But, a naturally-aspirated engine with a 10:1 static compression ratio at sea level would have an 8.0:1 effective CR at 10,000 feet.

So, the bottom line is that being "in the boost" can actually increase the efficiency of an engine, as Tom reminded me.

 

That sounds good and all and what you explain seemed to be the case when I had my 2007 Supercharged Mustang GT. I am not an expert on the subject by any means, however I am just reporting what I have been seeing as "real world" results in my area (Washington State).

It seems that the guys around here that I know with Ecoboosts (about a dozen or so and all 2013 and older mind you) are all complaining about their gas mileage and when I ask them what they get they are averaging the same or lower than my 2015 5.0L.

After a couple complained I made it a point to ask anyone I talked to with one just to see. Yes there are other variables to it (i.e driving style, towing, etc...). I personally haven't grown out of my Mustang Lead Foot so I know I could easily get better gas mileage if I tried.

Right now on long trips down the highways up here I'm getting 22-23mpg in my 5.0L which is respectable, but around town on the island I'm on it drops to about 15-16 because everything is about 35mph average.

 

I've yet to see a whole lot of evidence that the 3.5EB gets much for fuel mileage.
It seems it can get decent mileage in certain situations but the posts I've read here and on other forums seem to indicate the fuel mileage isn't that great.

 

I think when you factor in the capability of the 3.5, the real world mileage is somewhat impressive. I've driven a 4x4 cc at 80 mph on the interstate and got just under 18. My 7.3 same configuration won't touch that at 80. No, not fantastic fuel economy, but when you get into the trucks that do better mpg wise, the towing/hauling ability drops off pretty quick.

 

Yup. My computer-guesstimated average for the first 400 miles is up to 17.4 MPG. And it is rising as the few miles I've driven are added. In fact, my calc's show yesterday's 250 mile jaunt must have been 17.7 to pull the average up as it did. Agreed it isn't the 19 the sticker suggests I'll get on the average, but it is headed there.

And yet it'll tow anything I'm likely to ever want to tow, carry anything I'm likely to want to carry, and do so in luxury and with 5-star safety. Compared to Rusty's 12.5 MPG and no creature comforts and little safety, I feel like I made the right choice.

 

When I picked my 3.5 EB, I didn't choose it for mileage. I bought it for the torque, and I'm still 100% satisfied with it.