Have a 2010 FX2 with the 4.6 3v. Locally (~750' above sea level) on 87 octane I get around 17.5 mpg per the computer. Went to Yellowstone and down through Colorado and in the mountains and high altitude on 85 octane I got around 19.8 mpg. No tune on the truck and unsure why better mileage at higher altitudes and pulling grades that here on relatively level ground. Any thoughts.

 

I experienced the same thing with my Bronco going across the forest roads from Chama NM to South Fork Colorado. I got a real nice 24.5 mpgs in 4 wheel drive and at 14000'. I'm going to assume it's the oxygenates in the gas that creates a lean burn and actually yields a little more aggressive timing curve. Don't know about yours but mine ran like crap once out of the higher elevations until I refilled the tank.

 

Your tire pressure increases at higher altitudes as well decreasing rolling resistance. Probably minimal but it helps....

 

If you were in Yellowstone ....top speed limit of 45mph....that could've contributed to your gain.....
Also in colorado they get some crazy wild dyno numbers over what us flat landers get...