Autoexpress magazine used the MIRA testing circuit in UK with a stock Vauxhall Astra. They tested various things among which was aircon vs opening windows.
They did a few laps then measured exactly how much fuel they could squeeze into the fuel tank before moving onto the next test giving very accurate figures.
They proved that up to about 60mph its worth opening the window rather than using AC and above this speed using AC.
They also tested roof bars, roof boxes etc with surprising effect.
DHA 
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