Dear Umbra,

Is it more efficient to drive down the highway with my windows down or the air conditioner on? I know the air conditioner decreases gas mileage, but I bet having my windows down at 75 miles per hour produces pretty good drag.

Thanks,
Sara

Dearest Sara,

This is July’s Question of the Month for Grist readers, apparently. I received a passel of virtually identical queries with minute variations. (“How much drag do you get from a drooling dog?”, etc.) Car air conditioning has obvious benefits, especially if you have a long commute in a sprawling all-road-all-the-time Southern city. With AC, you can pretend you aren’t in the bowels of hell, although the delusion will vanish when you have to step outside to pump your gas.

And rest assured that if you crank the AC, you will have to pump your gas more often. The physical weight of the air conditioner adds to the drag of the car, and running it takes power from the engine; both reduce your gas mileage. There are other environmental implications to car air conditioning as well: With a few exceptions, auto air conditioners use chlorofluorocarbons, which can be released into the atmosphere during maintenance. So all in all it would be best to not even have AC in your car. If you must use the AC, it will be most efficient at a constant engine speed and low fan. [Amended, August 23: Oops. I goofed. CFCs were banned from U.S. car air conditioners in 1995, so newer vehicles do not pose a threat to the ozone layer, whatever their other environmental implications may be. For further clarification and more debate about windows versus AC, check out the Letters to the Editor section of Grist.]

Still, rolling down windows wins the efficiency contest hands down. Open windows will cut your fuel efficiency by about 2 to 3 percent; the air conditioner will cut it about 15 percent. Of course, these numbers come from tests done with a specific car, and your vehicle may be different. You could easily conduct a personalized test, dog and all.

Until then, here’s the textbook procedure for surviving car trips in August: Tint your windows, park in the shade, put one of those windshield covers on, leave, come back, air out the car, drive off, cool the car by opening your windows until you get on the highway, then roll up the windows and open the vents. Don’t forget to remove the windshield cover.

And one last thing: Driving your car at 75 miles per hour is itself fuel-inefficient. It’s better to toodle along at 45-60 mph.

Happy trails,
Umbra