Actually, my anti-Prius (see below) advises in the owners manual to roll the windows up and use the AC. The aero drag is worse than the AC drag. At highway speeds of course. That is, above about 80 km/hr - 50 miles per hour.
I tested it! On one road I frequent in summer there is a long mountain decent. The automatic will not hold the vehicle's speed to reasonable levels, in 3rd gear. 2nd gear is too slow. I tried turning on the AC. No difference, even when I lowered the climate control target temp. Opened the windows, and voila, it held the speed. Then I had to fiddle with the window open % to stop the "thumping" you get from air pressure resonance. It's a long decent, about 10 km, so I had lots of time to play.
