Does running the AC use more gasoline?

Dear Car Talk

Dear Car Talk | Sep 01, 1996

Dear Tom and Ray:

Is it true that one uses more gasoline while running the air conditioner in
the car? I have a friend who insists on driving with the windows open in
order to save on gasoline usage. Isn't it a health hazard to drive with all
the windows down and inhaling all the gas fumes from other cars? Please
advise! -- Jenny

TOM: Gee, Jenny. When I drive with my brother, the hazard is inhaling the
gas fumes from INSIDE the car -- especially after he's eaten at Joe's Red
Bean Burrito Emporium.

RAY: Actually, the fumes of other cars are not much of an issue in this
case. Whether the windows are open or not, fumes and outside air are still
entering the passenger compartment through the vents. That allows fresh
oxygen in even when the AC is on.

TOM: And your friend is right that you do use more gasoline when you run
the air conditioner, Jenny. Why? Because it takes energy to run the air
conditioner, and that energy comes from the engine. And when you ask the
engine to do more work, it uses more gasoline.

TOM: But here's the good news, Jenny. Believe it or not, at highway speed,
driving with the windows open uses even MORE gasoline than driving with the
AC on and the windows closed. How is that possible? Because on modern cars,
opening the windows basically ruins the aerodynamics of the car. It makes
it so much less "slippery," that none other than the Society of Automotive
Engineers says you'd be better off with the air conditioner on and the
windows closed.

RAY: What's the difference in miles per gallon between windows-open and AC-
on-windows-closed? Probably not a whole lot. So it's really a matter of
personal preference. But you can certainly use that argument on your friend
and get her to close the windows -- at least on the highway.

TOM: But if she suddenly takes up smoking Tiparillos to get back at you,
don't blame us!

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