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.

Consumer Reports' auto-test department reports that the air conditioner reduces your car's fuel efficiency by up to 10 percent. So to achieve maximum fuel efficiency, motorists should avoid using the air conditioner at speeds below 40 mph and travel with their windows down, explains Gabe Shenhar, senior auto test engineer at Consumer Report's auto-test department.

"But as your speed increases to 45 mph, or highway speeds," says Jason Toews, co-founder of GasBuddy.com, "wind drag becomes an issue. Driving with the windows down increases the drag on your vehicle, resulting in decreased fuel economy by up to 10 percent. Drive at speeds over 55 mph with windows down and you'll decrease fuel economy by up to 20 percent or greater."