Meh. I'm highly skeptical of these stratospheric MPG claims, particularly for the 3.5. I drive like an old grandma and it's tough for me to break 16 MPG. Of course, most of my trips are short city trips but even on the highway, 18 is the best I see on a regular basis.
The only way I can figure people are getting 20+ MPG on a regular basis is that they have a long commute that features a lot of freeway time, is relatively flat and there is no wind. Any kind of stop-and-go, any kind of climbing, or any kind of wind will knock MPG down quickly, at least that's my experience.
I'm on a couple of FB groups and people on there are always bragging about 20+ MPG numbers. I can't help but wonder if most of them are running 2wd trucks on factory tires and driving 50+ miles of flat, windless freeway every day.
Of course a lot of it depends on how YOU drive and the driving conditions you mention, especially the stop-and-go. Mountains don't seem to affect the mileage on my truck that much - seems like the mileage I lose going up a mountain pass is regained going down the other side...I live in Nevada where there are a LOT of high mountain passes. Those turbos laugh at mountains! I think it must have a lot to do with the individual truck also.
I'm one of the lucky ones. 2016 F150 Supercab bought new in May 2016. 3.5 Ecoboost, 4x4, 3.55 gears, wearing LT265/70R17 load range E tires. This truck has been outstanding in every way. Oil changes and tire rotations are all its needed since new. As with all my vehicles, I maintain a log book to log everything, including fueling. Periodically I enter all the fuel data into an Excel spreadsheet which calculates the fuel mileage exactly. With nearly 58,000 miles on the odometer now, the overall fuel mileage according to the spreadsheet is 20.66 mpg. Looking at my spreadsheet, my absolute worst mileage was 13.8 mpg recorded when towing 4,000 lbs into a fierce headwind. The best mileage was 24.09.
I think it helps my mileage that I live in an area with very few paved roads - but miles of very good dirt roads so I do a lot of driving at 45 - 60 mph. Shots taken at dawn yesterday:
That's an 800 lb Yamaha Kodiak in the back, plus there's a lot of gear on the back seat. When I got home last night my lie-o-meter was indicating 21.8 mpg but since it is almost always optimistic I figure the actual mileage is closer to 20.8.