While on vacation in Wyoming and Colorado, my 5.4 Liter F-150 got 20 mpg over 3 thousand miles of driving at highway speeds and continuously at altitudes above 7000 feet . For the previous 90,000 miles at lower altitude it only got 15-16 mpg. Any ideas as to why? I got 100+ miles more per tank of fuel than I ever got at lower altitudes. I am now back at 400 feet and 16 mpg.

My son's 2000 F-150 also averages about 16 mpg at the lower local altitude. I assume that is about what to expect normally.

 

I believe that it has to do with the density of the air. The thinner air up there is easier to move, so the engine can suck it in easier and the wind resistance on the front of the truck is lessened.

 

Air/Fuel Ratio - For a constant A/F ratio you decrease the amount of air (high altitude) you also decrease the amount of fuel used. Of course less fuel means less power.

 

I really didn't notice a great deal of power loss at altitude, although I had expected it after having driven at these altitudes before. The previous thread about less aerodynamic drag may be valid, but I really think the air fuel ratio at sea level is probably not as correct as it appears to be at altitude as the computer adjust for the altitude. I wonder if there is a curve the computer follows for a change in air density and if there is some way it could be available for review?

 

It has a barometric sensor in the MAF too. It sensors pressure and the volume of air being drawn into the engine.