Staying Cool and Saving Gas: A/C or Open Windows?

With American drivers suddenly looking for every way to save on the cost of driving, I thought I would revisit an old question, “which is more efficient, air conditioning or open windows?”

I first ran into this question while in college at the University of Michigan. I had a roommate who had heard that the refrigerant used in air conditioning systems was bad for the environment so he went the whole hot and humid summer without turning on his car’s air conditioner.

Of course, he didn’t realize that Freon (this was before it was replaced with R134a as a refrigerant in cars) isn’t actually released into the atmosphere when you run your A/C. Air conditioning runs in a closed loop and the environmental concern about the chemicals comes when some is accidentally released during servicing or if the system develops leaks… ironically most likely to occur after long periods of disuse. Running the A/C system distributes lubricating oils to the seals in the system so an unused air conditioner could develop leaks.

Of course, when faced with this information he changed the topic to say that it was more efficient to drive with the windows open because air conditioning makes the engine work harder. Ah, perhaps he had a point? Frankly, I didn’t have an answer.

Years later, I was working on the GM EV1 (electric car) program. If you didn’t know, this was the most aerodynamic production vehicle ever offered for sale with a coefficient of drag of 0.19. Since the car carried so little energy on board (the 1,200 lbs of lead-acid batteries provided the energy capacity of about ½ gallon of gasoline… providing about 60 miles of driving range) every step was taken to minimize the use of that energy. Besides weight savings to conserve energy when accelerating, air drag is the biggest, well, drag when driving at speed. As an example, I was once told that they didn’t use a mast-style radio antenna because it would have cost nearly a mile of range.

Anyway, we often had to drive the cars around Los Angeles and Phoenix for various events and the question arose whether we were better off using the A/C or opening the windows for maximizing range. There was no hesitation from the engineers: They said that we should make ourselves comfortable and use the A/C with closed windows when driving over 55 mph. I wish I had this information at hand when I was sweating in my roommate’s car on a 90 degree Michigan day!

Why is this the case? Well, while running the A/C does put a drag on your efficiency, running with the windows down does as well. It isn’t that simple, though. The aerodynamic drag varies with your speed. Air drag increases exponentially as speed increases, so driving 75 mph requires much more energy than driving 55 (sorry, Sammy Hagar).

At freeway speeds, this drag becomes greater than the cost of running the air conditioning compressor.

While perusing the Web, I came across a study by the Society of Automotive Engineers (http://www.sae.org/events/aars/presentations/2004-hill.pdf) that looked at this exact question. They found that at high temperatures the cost of running the A/C was between 5% and 10%. However, the drag caused by running with the windows down at 68 mph (110 Km/h) varied from 8% to 20%.

The lower number was for SUVs… basically, cars with bad aerodynamics in the first place suffer less from the effects of opening the windows. The 20% number was for a regular sedan. Though they didn’t disclose what type they used, they do indicate that it was a large sedan with a V8 engine. A really aerodynamic car like a Honda Insight or a sleek coupe would be affected even more.

So the bottom line is that around town you are better off with the windows open but once at speed you should close your windows even if it means turning on your air conditioner.

If you drive a Hummer or a Mercedes G-Wagon with the aerodynamics of a brick… well it doesn’t really matter. Just go buy Exxon stock.

Of course, driving while suffering from heat stroke isn’t good either, so here are some other hints about staying cooler if you must run your air conditioner around town:
• Try to park under cover or use a window shade. The cooler your car is when you get in the less you’ll need to run the A/C to cool it down.
• In the same vein, if your car is hotter inside than the outside air, open your windows before turning on your air conditioner.
• Max A/C (a.k.a. recirculated air) isn’t always the best way to cool down. Again, if the air inside of your car is hotter than the air outside, you’re just running super-heated air through your A/C system. Use the fresh air setting until the interior temperature is lower than the air outside.