Q. Recently, I heard that turning off the car's air conditioner and opening the window will increase gas mileage and thus save money. While I'm all for saving money, this idea sounds bogus. I say opening the windows would increase the drag, and therefore reduce your gas mileage. Am I right?
RAY: You are right, Sam. Using the air conditioner does lower your gas mileage, but not as much as opening the windows does.
TOM: Now, just to be clear, we're talking about driving at moderate to high speeds here. At those speeds, where aerodynamics play a crucial role in your mileage, you're always better off with the windows closed on a modern car.
RAY: When they design cars now, and do wind-tunnel testing or modeling, it's always with the assumption that the windows will be closed, so the sides of the car will be smooth. Otherwise, the airflow around the car is interrupted, and mileage will suffer. That's why they never let you roll down the windows on those 757s.
TOM: Right. That, and the frigid temperatures and the lack of oxygen. Anyway, at lower speeds, it's more of a toss-up. Imagine if you're stopped in traffic, or moving at less than 20 miles per hour. Then, aerodynamics is hardly a factor. So, in that case, turning off the AC and opening the windows would save you more fuel.
RAY: Of course, when you're stopped in traffic and open the window, there's no what? Breeze! So it's not always a good option, despite the energy you'd save.