AC effect on fuel economy

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Most people think this number is too high, but in carefully controlled tests by the EPA, the average effect from running the AC (on a wide variety of cars) was found to be a 20% hit on fuel economy. Of course driving with the windows open at highway speeds, also hurts fuel economy due to the added aerodynamic drag of the open window.
The set- temperature’s effect on how much the AC uses is highly-dependent on the design of the system. Most automotive AC systems control temp by mixing-in warm or heated air into the delivered air, and so the set temp has little effect on how often the compressor runs. More modern systems run the compressor less often or in low-power mode (if the compressor is variable-displacement or electric (like in an electric or hybrid car) if the set temp is higher. the AC fan speed probably has a higher effect on how often or hard the compressor runs, because even in an old-fashioned system, the compressor shits off when the evaporator system gets close the the freezing point. That will happen more quickly at low fan settings.

Overall, AC use is the most sensible way to stay comfortable at highway speeds, and open windows can save significant fuel at lower speeds.

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