A/C or windows down? Which Saves more gas??

As the weather warms up, it's interesting to see some people driving around with their windows down and sunroofs open, while others do the exact opposite and run their air conditioning. But which option is more fuel efficient? After all, A/C eats a bunch of gas. But having the windows down has got to create more drag on the vehicle. It turns out that both of those are true, and for that reason it's most fuel efficient to use your air conditioner at speeds over 45 mph, since wind drag would increase to where it is far less efficient to have the windows down. But under 45 mph, it's more efficient to have the windows rolled down since the amount of drag is not significant enough to be worse than air conditioning.