Best highway option: A/C, or rolling down windows?

[Red SVX 92]

I was just reading an engineering textbook, and a sidebar text section mentions tips for minimizing gas consumption. I'd quote the exact statement, but I'm lazy right now, so I'll paraphrase. Basically, the book said that at highway speeds, turning on the air conditioner to achieve a cooling effect was more fuel-efficient than rolling down the windows and sunroof, and noted that this was the case especially in newer, more aerodynamic vehicles. Now, I know that this is just a generalization, and may not apply to the SVX, but I'm curious: would anyone have any substantiated evidence or knowledge which proves or disproves this statement? I usually drive on the highway with the A/C off and the windows and sunroof down, because I believed that I was saving gas. I like the feel of the wind, but I'd rather have A/C, a quiet passenger cabin, and no funky cow manure smells if it saves gas.