Having the Aircon ON in a modern car, uses LESS fuel than having all the windows open, which creates drag.
And while it is true that you should always keep your windows closed for it to work at its most efficient......remember that you gotta blow air through air vants and ducts inside your dash board that are HOT AS AN OVEN.....
So first thing you do, is switch on the Aircon, OPEN the windows, blow it flat-out for 30 seconds, THEN close the windows.
I have been experimenting all day long