Dear Tom and Ray:

I appeal to your expertise to settle a discussion a friend and I had recently. He says running the air conditioner in a vehicle uses as much as two gallons of additional fuel per mile. He prefers to travel with the windows down. I concede that using an air conditioner may reduce gas mileage, but I think his estimate of fuel use is high. I also think that when you run a vehicle at highway speeds with the windows open, you change the aerodynamics and increase wind drag, which also lowers fuel economy. Since we live in a hot weather climate, and summer is fast approaching, your answer will determine whether we have a year round friendship, or just a cool weather one, as I like to arrive at my destination cool and neat rather than hot and windblown.
Lois

TOM: We're siding with you, Lois. Studies have actually been done on this very issue, and the results show that driving a modern car with the windows down uses MORE gas than driving with the windows up and the air conditioner on.

RAY And your friend IS way off on his estimate of fuel use. Sure, it does take some power to run an air conditioner, and that power does come from the engine (and therefore uses some additional gasoline), but it's more like one or two miles per gallon....not two gallons per mile!

TOM: And apparently, it takes even a little more fuel to overcome the increase in wind resistance that comes from driving with the windows wide open.

RAY: Then, as you well know, Lois, when you factor in the increased hair-styling and dry cleaning bills caused by that open air driving, air conditioning begins to look better and better.