Influence of window opening vs. A/C use on fuel economy

Kind of a late reply...

I didn't see the Mythbusters test. My van doesn't cycle the AC - you
are supposed to blend in some heat for comfort. In moderate weather,
I will turn it on and off manually rather than blending in heat. In
really hot weather I run it full time for comfort so I don't think an
automatic control would save much there.

According to my results, rolling down the window only costs about 2%
on fuel mileage, so the AC would have to get down to a 33% duty cycle
to break even. Perhaps newer cars are more efficient on AC.