Q--I have a 2006 Chrysler 300C with the HEMI engine that uses the cylinder deactivation system to save gas on the highway. Recently I drove to
A--Fuel economy improves at higher altitude in all engines because the pumping work is reduced. As you go up in altitude, a lower manifold vacuum is required to move the car. The lower vacuum means lower pumping work and the result is better fuel economy. This also explains the reduced power. Think of an engine as an air pump and you may start to get the picture.
Since you were driving at about 80 mph most of the time, the cylinder deactivation would not kick in, so that is not a factor.
Regarding leaning out the fuel mixture, you are probably old enough to remember when carburetors needed to have smaller jets installed at higher altitudes to keep the engine running right. They were not leaning out the fuel mixture, but lowering the amount of gas delivered to more closely match the amount of air. Today, the computer maintains the air/fuel mixture at precisely 14.7 pounds of air to 1 pound of fuel (stoichiometry) no matter the altitude. It’s not leaner; it’s just right.