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In a gasoline powered car at high altitude, less air means less fuel burned but it also means less EGR. This all translates to less trapped mass in the cylinder and more heat of combustion going into the engine block and head. At a lower altitude the ECU would compensate by increasing EGR flow to increase the trapped mass in the cylinder. This extra mass will absorb heat that would otherwise heat the block and head and allows more work to be extracted by the piston.
So at higher altitudes your economy drops.
EDIT: Also inert gas injection will work better than EGR. Gases like helium and argon will raise the specific heat ratio of the combustion gasses during the power stroke. This results in more power being extracted by the piston. This is not cheap however.
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