I don’t drive much, and I live in San Francisco where it’s cold in the summer, so AC is not a factor in my everyday life.
When I do drive on occasion out of town and to warmer climes, I agree with this, from a NYTimes article, The Air Out There: An Endless Windows-vs.-Air-Conditioning Debate.
“The rule of thumb is to keep the windows down while on city streets, then resort to air-conditioning when you hit the highway,” said a July 2008 “Green Lantern” column.
According to the article, it’s an endless debate. As with many of these Everyday Sustainable concepts, there’s no simple answer. I say, drive less and be aware or your decision to AC or not.