Fuel Economy and A/C

May 18, 2008 at 11:48 am | | everyday science, news, science and the public

So, I’ve heard from several people over the course of my life that it is more fuel efficient to run a car with the windows down rather than use the A/C.

However, a new article on Fuel Economy Myths from CNN-Money suggests that this isn’t so at high speeds.

There’s no question air-conditioning makes extra work for the engine, increasing fuel use. But car air conditioners are much more efficient today than they used to be. In around-town driving, using the A/C will drop fuel economy by about a mile a gallon.

Meanwhile, driving at higher speeds with the windows down greatly increases aerodynamic drag. As speed increases, drag becomes more of an issue, making A/C use the more efficient choice at high speeds.

At most speeds and in most vehicles, A/C use drains slightly more fuel than driving with the windows down, contends David Champion, head of auto testing for Consumer Reports. “My final take on is that it’s very close,” says Phil Reed, consumer advice editor for Edmunds.com. “It’s hard to measure the difference and every vehicle is different.”

The best choice—if temperature and humidity allow—is to keep the windows rolled up and to turn the A/C compressor off. You can keep the fans running to blow in air from the outside, but your car will be as aerodynamic as possible while still letting you breathe. You will save gas, but the fuel economy improvement will be slight.

Other myths include the idea the Premium gas gives you better gas mileage (false in modern cars – the car’s computer can tell the density of the fuel and adjusts the spark plug timing. Lower-octane fuel “slightly” decreases horsepower, but has negligible effect on fuel economy), and that over-inflating tires increase fuel economy (obviously true – less friction, but dangerous due to braking and turning issues).

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8 Comments »

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  1. in my car (a 95 toyota tercel), AC uses a lot more fuel than open windows on the freeway. i’ve tested it and it’s very obvious. newer cars may be different.

    Comment by sam — May 18, 2008 #

  2. As a broke-ass undergrad driving his dad’s old truck (an ’89 ford pickup truck) in the blazing Arizona summers, I learned that there is no limit to the money I’d pay for A/C. I would whore myself for the money, it didn’t matter. “It’s a dry heat” stops being an excuse above 105°.

    Comment by excimer — May 18, 2008 #

  3. They did this one on Mythbusters, and if I remember correctly, they found it to be a draw.

    Comment by Kirk — May 18, 2008 #

  4. no, windows down beat AC on by 30 laps. see here.

    Comment by sam — May 18, 2008 #

  5. “Meanwhile, driving at higher speeds with the windows down greatly increases aerodynamic drag. As speed increases, drag becomes more of an issue, making A/C use the more efficient choice at high speeds.”

    I don’t think 45 mph is “higher speed.” I would imagine they meant 55 or 65 mpg – I’d be interested to see what happens then. As one commenter on the Mythbusters site correctly said, drag is proportional with the c*v^2, where c has to do with the Area, drag coefficient, density of air, etc., and v is the velocity. So 10 more mph is more than 100x the drag, which is then multiplied by the change in area from having the windows open.

    An interesting experiment would be to test the velocity dependence of the A/C vs. Windows fuel consumption, to see at what velocity A/C wins over open windows. I don’t think 1 data point at relatively low speed (much slower than what anyone would drive on the highway) conclusively busts this myth. And we ARE talking only about high speed – everyone agrees A/C uses more gas in city driving.

    Comment by charles — May 19, 2008 #

  6. yes. however, the only two data points—my tercel at 65+ and an SUV at 45 mph—point to AC being a bigger drain on the engine than windows open. presumably, there is some speed where windows-open drag will be a bigger drain than AC, but on some cars, that velocity is not achievable.

    i’m sure that the tipping point varies from car model to model (because of shape and AC efficiency).

    Comment by sam — May 19, 2008 #

  7. Do you know where I can find actual data from an experiment(s) comparing fuel economy with windows down vs. air conditioner on. I am an AP Statistics teacher and I would like to use this data in a project. Thanks.

    Dave

    Comment by Dave — May 26, 2010 #

  8. In my car with the windows down I get less fuel mileage no mater how fast I drive or how slow I go.

    Comment by tim — July 16, 2012 #

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