Generally speaking, if you drive <70km/h, switching off the AC and winding down windows saves more fuel but if you cruise on a highway >70km/h, switching on AC with windows winded up will save more fuel. This is simply because >70km/h there is more air turbulence with your windows winded down.
My best experience is to drive in less polluted countries like New Zealand and Australia when temperature is moderately cool, no need to wind down windows, no need to switch on AC, just let the outside cool fresh air enter the cabin!
