I don't even run the fan when I have my windows cracked; the suction pulls plenty of air through the vents.
Check this:
Fuel Efficiency: AC On Vs. Windows Down - Business Insider
Quote:
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The researchers used a wind tunnel and laps on a track to test a sedan and an SUV at speeds of 31 mph, 50 mph, and 68 mph, in 86 degree F weather.
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