Mit talaltam:
"A little footnote here : assuming all other factors are the same - same octane of petrol, same filters, same weight of vehicle etc., you lose about 3% of your engine's horsepower for every 300m / 1000ft above sea level that you drive. Worth knowing next time you go carving up the Stelvio Pass in Italy. At 9000ft, you're a full 27% down on engine power compared to sea level."