The thought that using the AC is more expensive in an economy (or hybrid) car versus a larger car (such as the Expeditions used on Mythbusters) is misleading via the way we compute fuel consumption -- miles per gallon, rather than gallons per mile. The AC will consume power (and thus gasoline) based on difference in temperature, number of passengers and sun intensity, and between different cars is effected by interior volume, window size and position, color of car, and window tinting, but it's effect is best measured in gallons per hour. Speed and distance have a minor effect.

Lets say that the hybrid AC consumes .25 gallons/hour, that it gets 40 mpg at 60 miles per hour. Thats 1.5 gallons/hour. Adding the AC gives 1.75 gallons/hour and converting back we get 34.3 mpg. That's a loss of 5.7 mpg, or 16%.

Lets say the big Expedition, which has twice the interior volume, has an AC that consumes .5 gallons/hour. It gets 12 mpg at 60 miles per hour. Thats 5 gallons/hour. Adding the AC gives 5.5 gallons/hour and converting back we get 10.9 mpg. That's a loss of 1.1 mpg or 9% which sure sounds much better.

But if we look at gallons per hour (or gallons per mile) we find that, of course, the AC is more costly in the Expedition, in fact twice as costly.

You can also see that the faster you go, the less the AC costs per mile. If you open windows, the drag will cost more at higher speeds. So there is probably a certain speed that represents the crossover point. I'd guess that in town the windows should be open and on an open highway they should be closed.