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Considering a few things here...
At higher altitudes, i.e. 5000 to 10,000ft ASL, there's less oxygen. The max power of a vehicle decreases at higher altitudes because the engine can't supply enough oxygen and must supply less fuel to run efficiently. Effectively, an engine rated for 110HP is cut to a lower level such as 70HP at say 8000ft ASL.
At the same time, air is thinner at higher altitudes, so drag (friction) is lower. The power needed to overcome drag at 50mph is less at 8000ft ASL that at sea level because the air there is about 1/16 as thick.
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