I'd heard that if you are driving down the freeway in the summer, you use less gasoline if you roll up the windows and use your car's air conditioner to cool off. The reason for that is the extra drag you put on the car by opening the windows at highway speeds uses more gas than the air conditioner. But I didn't know if that rule applied to short hops to the grocery store through a school zone, too. Now I have the answer.
According to AAA, the magic number is 40. So, if you are driving over 40 miles per hour, use your car's air conditioning. If you are driving under that, you'll use less gas if you open the windows to cool down. What you do if you're cruising at a steady 40 mph is up to you.