#46: Neutral and Impartial Answer

The Question

Jen says:
2005-08-17 17:03:12

Oh great and all knowing Shaun…

For some time I have been under the impression that you should limit the use of your car air conditioner because it uses more gas. However, yesterday on the news I heard that in reality this does not work. They claimed the having your windows down creates extra drag on the car, which therefore negates any savings. What is the real story?

Also, some crazy guy in my life always insists that I leave my air conditioner on all year. The idea, I believe, is that it will produce a dry air, which is beneficial in the winter. Is there actually a benefit? And…am I using more gas than necessary by doing this in the winter, when my windows are up? Any clarification that you can offer as a NEUTRAL researcher would be greatly appreciated as we attempt to settle this issue once and for all.

The Answer

Two questions in one! How economical.

To answer the first question – the simple answer is “It depends”. What does it depend on? Speed.

You see, if you are going relatively slowly (under highway speeds), it is more economical to leave the windows down and not run the A/C. The A/C, while it doesn’t use gas itself, uses power from the engine (approximately 8 horsepower), and the production of that extra horsepower requires extra gasoline. When your car is moving relatively slowly, the extra horsepower needed to run the A/C outweighs the horsepower needed to overcome the drag from your open windows. (Leaving the windows down renders your vehicle less aerodynamic).

HOWEVER, when you are on the highway, and moving relatively quickly, the drag from having your windows down OUTWEIGHS that of the A/C unit’s operation. It’s actually better for your gas mileage to run the A/C in this case than to have the windows down. Good news for the hot highway traveler! Of course, some of us don’t HAVE A/C in their cars…

On to question 2.

This crazy guy is ALMOST right. But not quite. I’m afraid you’ll have to bow to his superior wisdom in a number of areas, but not overall. The fact is, regardless of the season, when you are trying to defrost or defog your windows in your car, the A/C is the way to go. A/C performs two operations on the air – it cools it, and it dries it. It’s the dryness that makes this the way to go. When you are trying to remove the moisture from your car windows, you will be far more successful if you blow dry air (which evaporates the moisture much more efficiently) than moist air. In fact, in many vehicles with A/C (unlike some of us who aren’t so lucky), the A/C goes on by default whenever the defogger is selected.

However, this doesn’t mean that you should leave the A/C on all the time, since you will be burning that 8 horsepower worth of gasoline. Use the A/C in the winter only when you have fogging issues. The rest of the time, save your gas by leaving it off!

Now, if that isn’t impartial and neutral…well, trust me, it is.

9 thoughts on “#46: Neutral and Impartial Answer

  • 10/10/2005 at 9:16 am

    Shaun, I must dispute your answer to this one. I disagree and say having your windows open is better than running the a/c, except at ridiculous speeds. I have proved through testing that at 75mph the a/c still uses more power than parasitic drag from open windows. I go up this certain hill at 75 with my a/c on, and I can’t make it all the way without downshifting. Next, I go up the hill at 75 with the windows wide open and I fly all the way up at 75. I have tried the test multiple times and get the same result.
    Here’s another way to look at it:
    Driving 60mph requires about 18hp to overcome drag for the average car. Now, lets open the windows. Do you really believe they will rob you of more than 8hp (a/c power) or 44% of the drag of the entire car?

  • 10/10/2005 at 11:09 am

    Wouldn’t this be controlled directly by the change in your CoD with open windows? For instance, I know in my VW (CoD .34) it’s better to open the windows unless I’m on the highway. But in the Prius (.29 CoD) I know it’s much better to rock the A/C, mostly because the car tells me average mpg and moment by moment mpg and I was bored and did the tests.

    So is there really a car independant answer to this question?

  • 10/11/2005 at 9:06 am

    I would like to turn your attention to Mythbusters Episode 22, where they did this test. While their methodology isn’t car independent, since they used only one car, it was a car that probably has a CoD much higher than either a VW Golf or a Prius, a Ford Explorer was used if i remember correctly. They only investigated highway speeds.

    They used two “identical” exploders and very precisely metered the gas. Then drove each at a constant speed for an entire tank (don’t remember it was either 55 or 65) the exploder with the windows down ran out of gas well before the A/C did.

    While it may depend on the car, I think it matters more when the car was built than what car. I’m guessing most cars made after 1995 perform better with the AC on rather than the windows down, at highway speeds.

  • 10/11/2005 at 11:31 am

    Mythbusters are good, but not great. They typically overlook lots of details that can significantly change their results.
    Also, the longer the trip is, the larger advantage A/C has. Once the car is cool and you are recirculating cool air, there is no need to run the a/c compressor constantly at 8hp and it shuts off as necessary.

  • 10/11/2005 at 2:18 pm

    Sorry Mike G., the Mythbusters are indeed great (and not just because I have a huge crush on Tory).

  • 10/12/2005 at 10:53 pm

    Good timing on this answer! The new Mythbusters tonight was a revisited show, where they try to fix any mistakes made in old experiemnts. They revisited the car AC myth and determined just what we already knew. 45mph seems to be the threshold point where using your AC becomes economical. Way to go Shaun!

  • 10/13/2005 at 11:12 am

    In conclusion, let me say this: Yes, individual cars will have varying results. “Your mileage may vary”. (As Ben indicates in his comment, the CoD of the car will be vary the A/C efficiency point). The answer provided is more of a guide than geared to a specific make and model. Because of this, I consider Mike G.’s experience more of an aberration than a general rule.

    I’m glad that Mythbusters took the hint from Defy S. McQuaid and did a little further research.

    And thanks for the support, Becky!

  • 11/22/2005 at 2:43 pm

    I am sorry, the mythbusters episode was pure bunk.

    When the vehicle was traveling faster (the first part of the episode), the A/C DID WIN. They didn’t like the result, so they slowed it down 10 MPH and turned the A/C up full. They guy had to wear a winter jacket or something.

    They also used a BIG SUV the shape of a brick. That means the A/C unit is sized larger(for the volume) and opening the windows makes less impact on areodynamics.

    When I watched the episode, my jaw hung open as they progressed from error to bigger error. Outside intentionaly trying, I don’t see how anyone could rig the test worse!

  • 6/7/2006 at 12:19 pm

    I heard this on Car Talk: Some Ford cars, definitely certain years of the Explorer, cannot have the heat turned on at the same time that the A/C compressor is running. Due to some horrible design flaw, turning the heater on will cause the A/C compressor to stop turning, creating enormously excess wear on the belt. You might want to do some fact-checking on this, it could be complete baloney.


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