There's the old saw that leaving your windows rolled down creates an aerodynamic drag on your car, cutting down on fuel efficiency. And there's the notion that the fastest way to drain your gas tank is by running your air conditioning.
Don't believe either one.
In two separate studies conducted in 2005, the automotive Web site Edmunds.com and Consumer Reports compared the fuel economy of both a sedan and an SUV at highway speeds with and without air conditioning and how open windows affected gas usage.
What they found was no significant difference in fuel economy in either sedan or SUV under either condition.""
This was actually mentioned in another thread. The main issue is "significant." Some people here have done their own tests with their scanguage, and the drop is usually quite dramatic (at least by my standards).
It was my belief too that the article is a bit misleading on what it mentioned and did not mention. For instance no mention of general tune up and keeping your car in good shape but promoted turning your car off. Kind of ironic how the general public views these issues.
They also didn't mention some details about the "test" average speed and vehicle type. A significant gain to me surely would not be the same to the general public who on average get 20MPG or less in a SUV.
In my Camaro there is no difference at all running the air conditioner on the highway with the windows rolled up. Windows down and T-tops out I get 2-4mpg less depending on the speed. Most of that is from the t-tops being out.
I think anything with 8 cyl is not going to show as much change due to a/c running. Especially considering most engines are cammed for power so they are not really running that efficiently. I would like to see someone do a test comparing mpg vs speed for windows up no a/c, a/c on low/normal speed, and windows down. I would figure there is a speed that a/c will be better than windows down and it is probably around 60mph for most cars.