There is less air pressure and as a result less drag on the airplanes. The result is less fuel is required to maintain the speed.
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One of the main contributing factors is that air becomes less dense the higher you go, therefore drag on the airframe reduces. Less drag aids engine efficiency.
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Along with the other two answers about drag, the ratio of fuel to air remains the same. Therefore less dense air requires less fuel.
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Aside from the thinner air being less draggy on the airframe the fact that this air is also very cold also helps with engine efficiency.
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