Air Conditioning Affects Gas Mileage

August 10, 2010 at 5:00 am 12 comments

By Chad Upton | Editor

In most cars, the air conditioner’s compressor is powered by a drive belt on the engine. When the air conditioner is activated, the compressor adds resistance to your engine. That extra resistance means your engine requires more fuel to turn at the same speed.

In other words, using your air conditioner burns more gas than not using it. Modern cars have very efficient air conditioners, but this truth still stands.

On a hot summer day, you have to keep cool. Does that mean it’s more fuel efficient to drive with your windows down?

Yes and no.

The Society of Automotive Engineers performed a study that examined this question in detail. They performed wind tunnel and track experiments comparing a car and an SUV. With the windows down, the car was half as efficient at 50 mph (80 km/h) than the SUV’s at 30 mph (50 km/h). It’s clear that driving the car with windows down has a dramatic effect on fuel economy, but it affected the SUV even more, especially when a 10 mph (16 km/h) crosswind was added in the wind tunnel.

Consumer Reports found that below 40 mph (65 km/h), drivers are better off with their windows down and air conditioner off.

Jason Toews from GasBuddy.com found at speeds above 45 mph (70 km/h), “wind drag becomes an issue.” He says, “Drive at speeds over 55 mph with windows down and you’ll decrease fuel economy by up to 20 percent or greater.”

Myth Busters has also looked at this issue on a couple occasions. The first time around, their methodology was flawed, so they tried it a second time. The second time, they came to the same conclusion as Consumer Reports and GasBuddy, that windows down are more efficient than running the air conditioner at speeds less than 45-55 mph (70-90 km/h) and the drag at higher speeds makes the air conditioner the more efficient option.

Of course, results vary by vehicle.

There are many factors other than windows and air conditioning that affect fuel economy: drive train, power-train, body aerodynamics and tire rolling resistance. If you want to know exactly how it affects your car, you might try experimenting for yourself. Some cars have a driver information center that displays fuel economy, otherwise you could order an after market product, such as CarChip Pro ($75-$85), which may help you get that data. If you’re interested, some of these are explored in more detail in another post, The Ideal Vehicle Speed for Best Fuel Economy.

If it’s hot outside, you should probably start with your windows down anyway. This gives your air conditioner a bit of a break by clearing the extremely hot air out of the car first.

Do prefer windows down or the air conditioner on?

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Sources: SAE (PDF), MythBusters (Ep 22, Ep 38), Edmunds, Bankrate, Consumer Reports, Missouri Gov

Photo: Tomás Fano (cc)

Entry filed under: Automotive, Be Efficient, Be Green. Tags: , , , , , .

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12 Comments Add your own

  • 1. LJ  |  August 10, 2010 at 7:39 am

    One tool I own and can definitely recommend is the ScanGauge. You can see your real time MPG or GPH along with many other readings in real time. Great for testing things like this!

    Reply
    • 2. Elbyron  |  August 10, 2010 at 5:08 pm

      Thanks for the recommendation. I think I just might go buy one of these! Partly for the efficiency monitoring, but mainly because it can check and clear Diagnostic Trouble Codes. My stupid check engine light has been on for almost a year, and the mechanic charged me $200+ to check it (some kind of electronics failure) and run some basic tests – but he didn’t clear it. For $160 I can deal with this problem myself as well as be able to troubleshoot future engine trouble a lot easier.

      For any Canadians interested in the ScanGauge II, you can buy it at http://www.giffordautomotive.com/ordering.htm for $160 CAD including shipping.

      Reply
      • 3. Alan LaRue  |  August 20, 2010 at 10:50 am

        Though I don’t keep written, I do calculate my mileage each time I fill up. I drove a 1999 Saturn SL1 for 7 years (until a month ago) and found that running the a/c reduced the mileage by about 10% (to 33 mpg), whereas driving without the a/c, whether the windows were up or down made no discernible difference (about 36 mpg). My commute involves over 30 miles of interstate highway and only about 5 miles off highway, and typical speeds of 65-70 mph.

        Not having kept precise records, I cannot say whether having the windows up or down made any tenths of miles per gallon of difference, and admittedly I seldom had all four windows down, but it seems significant nonetheless that the mileage was essentially the same up or down.

        The fact that this car had only a 100 hp engine may be significant, as well. Perhaps some more comprehensive tests are in order: Cars of varying sizes, aerodynamic shapes, and with different sized engines, and comparisons with windows closed, driver’s-window-only open, both fronts, and all four. Only then will we get some real answers.

  • 4. Alan LaRue  |  August 20, 2010 at 10:55 am

    Follow up: Your quote Consumer Reports as saying that over 40 mph, the aerodynamic effect comes into play, but I’ve just read on their website that in tests with a Honda Accord, there was no discernible difference, windows up or down, at 65 mph.

    CR Article here

    Reply
  • [...] of of us are thinking about lowering our gasoline use, too many people are not aware that using the AC can reduce their motor vehicle’s gas mileage by as much as three miles per gallon. Once again, however, auto window tinting film provides a [...]

    Reply
  • 6. best gas mileage  |  October 12, 2011 at 11:08 pm

    Great information. I have been looking for something like this for awhile. I like this topic and I like how you used your in depth research to talk a bit more about it. Thanks for putting in the extra time. Much appreciated.

    Reply
  • 7. Fair Weather Fuel Economy « stewardsofearth  |  April 5, 2012 at 10:18 am

    [...]   Overall, put windows down at lower speeds, and use A/C at higher speeds. (A good blog over the subject.)  Anyway, here are some tips to keep in mind during warmer [...]

    Reply
  • 8. Ceola Bushard  |  October 10, 2012 at 4:33 am

    Air conditioners are great since they can make the humidity at a very comfortable level..“:.

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  • 9. Angelia Skill  |  March 10, 2013 at 9:56 am

    The first modern air conditioning system was developed in 1902 by a young electrical engineer named Willis Haviland Carrier. It was designed to solve a humidity problem at the Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn, N.Y. Paper stock at the plant would sometimes absorb moisture from the warm summer air, making it difficult to apply the layered inking techniques of the time. ^

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    Reply
  • 10. charles brennan  |  April 11, 2013 at 5:53 pm

    Once the ac is turned on, does adjusting the temperature dial make a difference in the mileage? For example, running the ac on the full cold/blue setting burns more gas than moving it down/a bit warmer?

    Reply
  • 11. Rey  |  July 16, 2013 at 6:25 am

    Hi guys, anyone can help me with my problem, since we are talking about ac compressor to engine, now a days I am experiencing this bad fuel consumption plus poor car performance “only” when the ac is turned on, every time the ac compressor kicks on I will feel the roughness and the acceleration is very poor. anyone could help me with this

    many thanks,
    rey (Dubai)

    Reply
  • 12. accounting services singapore  |  December 14, 2013 at 3:27 am

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