Since virtually all modern injection systems compensate for altitude, there should be little, if any difference in performance. Not so with a carburetor, since the thinner air at higher altitudes tends to make an engine run richer than it will at sea level. An aircraft engine with an altitude-compensating carburetor, or with a pilot who's good at watching cylinder head temperature and adjusting mixture manually will get better fuel efficiency at altitude.
With either a MAP (manifold absolute pressure) sensor, or a MAF (mass air flow) sensor, the fuel injected automobile should run at least as well at altitude as it does down lower, and you'll get slightly better mileage because the injection is running leaner to match the air density. Couple that with an oxygen sensor and a computer clock frequency that allows mixture adjustments up to 90 times a second, and you've got an extremely efficient system!
Jerry
