Will you get better gas mileage with your windows down or with the A/C on? How does this compare to windows up, or with the heat on? At lower speeds, it will be more efficient to lower your windows. At higher speeds, where drag increases exponentially, the drag created from having your windows down will have a higher impact on your efficiency, and thus it will be better to drive with the windows up and the A/C on. But what speed does this occur at? Let's run some tests to find out.
A/C vs Windows Down - http://youtu.be/ue6DsRncyfI