Gas Mileage Question

Discussion in 'Community' started by EJBasile, Sep 9, 2005.

  1. EJBasile macrumors 65816

    EJBasile

    Joined:
    Apr 20, 2004
    #1
    I was thinking about this on my way home today.

    If your driving on the highway you would obviously get better fuel economy if you turned off your A/C. But if you turned off the A/C and opened the windows of your car would the aerodynamics get messed up and reduce your fuel economy. If so by how much. It would obviously depend on the vehicle, but I'm just speaking in general.

    Maybe I'll preform an experiment one of these days...
     
  2. stubeeef macrumors 68030

    stubeeef

    Joined:
    Aug 10, 2004
    #2
    the mileage gained by turning off your A/C is very tiny if you have a newer car. The effeciency is so much better now.

    I would guess that over 40mph the drag would be worse than any A/C.
     
  3. lopresmb macrumors 6502

    Joined:
    Apr 29, 2005
    #3
    someone gave me this theory one time, I don't buy it. I can't imagine that you aerodynamics would be afected by opening the SIDE windows. There is no air that flys into it (maybe something to do with pressure flowing across it), but I can't imagne that it would be a big deal. I would assume, certainly not as much of a mileage user as an air conditioning system would be.
     
  4. saabmp3 macrumors 6502a

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    Tacoma, WA
    #4
    Click and Clack, the official source of this kind of information protest that using the A/C uses less fuel than the drag created by opening the windows (and really killing your aerodynamics).

    BEN
     
  5. Abstract macrumors Penryn

    Abstract

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    #5
    Yeah, you save petrol by using your AC, but that's only at higher speeds. At lower speeds, say under 60-70kph, or around 40 mph, it doesn't really matter if you use your AC or just open your windows. Don't bother opening your windows on the highway, though.
     
  6. aquajet macrumors 68020

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    Feb 12, 2005
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    VA
    #6
    I have a VW New Beetle, 2.0L, 4-speed auto. Recently drove across country. Did some mileage tests along the way...keep in mind all driving time spent on the interstate highway, cruise speed 75-85 mph.

    Windows up, A/C on: 31.1 mpg
    Windows up, A/C off: 31.9 mpg
    Windows down, A/C off: 27.7 mpg :eek:
     
  7. mad jew Moderator emeritus

    mad jew

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    Apr 3, 2004
    Location:
    Adelaide, Australia
    #7
    Interesting stuff. I've always wondered about this little theory. Thanks for the stats aquajet. :)
     
  8. EJBasile thread starter macrumors 65816

    EJBasile

    Joined:
    Apr 20, 2004
    #8
    I've tested the A/C off and I get about 2mpg better mileage on my Rover.
     
  9. devilot Moderator emeritus

    devilot

    Joined:
    May 1, 2005
    #9
    There's this awesome show called "MythBusters" and they tested this theory out-- it is true, using the AC kills fuel economy when compared to just having the windows down.

    Two SUVs were used; exact same specs, model, everything... Same exact amount of fuel put in and one driver blasted the AC the whole time while the other drove w/ the windows down. They drove around and around an open race track. Although the data was screwy nobody could deny the truth because the AC using SUV rolled to a standstill over a half an hour before the windows-down SUV did.
     
  10. skubish macrumors 68030

    skubish

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    Feb 2, 2005
    Location:
    Ann Arbor, Michigan
    #10
    Without a doubt, opening your side windows creates drag. It disrupts the flow of air around the vehicle. Any disruption like that can create drag. A/C is pretty efficient now, so there is not a big benefit to turning it off.

    The best benefit to fuel economy is to slow down. I read somewhere that you gain 10-15% fuel economy buy driving 60mph instead of 70mph.
     
  11. mactastic macrumors 68040

    mactastic

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    Colly-fornia
    #11
    I saw that one. Jamie sure was cold by the end!

    But I don't think they were as thorough as they should have been to definitively answer that question. They left out the speed variable, and they only tested it on an SUV.

    The reason I think it makes a difference is that when my wife's turns the AC on in her Civic with it's ~120 HP there is a noticeable drop in available power. If you really want to accelerate in that car you turn off the AC until you hit cruising speeds again. But in my truck with ~200 HP there is no noticeable drop in power. Thus I speculate that the effect of the AC might just be more pronounced on the smaller car because it takes roughly the same amount of power away from an engine to turn an AC compressor regardless of engine size.

    Who knows, that might just make the Mythbuster's proof that much more positive. But her car is also vastly more aerodynamically superior, so an open window might affect it more...

    I just don't know. I think you'd have to do some pretty extensive testing to say definitively.
     
  12. ham_man macrumors 68020

    ham_man

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    Jan 21, 2005
    #12
    Who run's their AC full blast? In reality? The only time I have ever done that is when it is 110 degrees outside...
     
  13. maya macrumors 68040

    maya

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    somewhere between here and there.
    #13

    O have not only seen people turn they A/C full blast, however they also had they window down. Yeah, tell me about it, totally weird. :rolleyes:
     
  14. Xtremehkr macrumors 68000

    Xtremehkr

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    Jul 4, 2004
    #14
    The manual for my Infiniti states that it is more efficient to use the AC to lower the temp at higher speeds. Opening the windows causes a significant amount of wind drag. Not only that, but the air is not much cooler in Phoenix during the summer, but I don't think they were referring to Phoenix specifically.

    I ended up getting a Toyota Tacoma double cab 4x4 instead of a Honda Rune. And there is nothing mentioned about opening the windows vs. using the AC. No matter what I do with the Tacoma, AC concerned, I end up getting about 19mpg. Which isn't all that bad. I am hoping for 21 with a CAI though.
     
  15. Abstract macrumors Penryn

    Abstract

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    #15
    Weird. Theory is that the opposite is true. Even tests I've read about have claimed that turning on the A/C and keeping your windows shut is the best way to save fuel at mid to high speeds, while at lower speeds it matters very little.
     
  16. After G macrumors 68000

    After G

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    Location:
    California
    #16
    I was always under the impression that A/C wasted more gas than leaving the windows open. I mean it takes a bunch of energy to convert refrigerant from a gas to a liquid state, and then to pump that refrigerant through your car to make the A/C work.

    I also thought that if the windows were down, the air in the car would flow out through the windows because it wasn't moving, while the air outside the car was, resulting in lower air pressure outside the car than in.

    Maybe someone can explain ... ?
     
  17. Xtremehkr macrumors 68000

    Xtremehkr

    Joined:
    Jul 4, 2004
    #17
    Modern AC systems in vehicles have become much more efficient. Opening windows, especially at higher speeds, creates a considerable amount of wind drag. Combine that with the ambient temperatures and it becomes more efficient to use AC, as the outside air may not be much cooler than the inside air. If there is humidity involved, using AC is definitely going to provide a much more comfortable climate.

    In my case, the color of the vehicle seems to play a role as well. A red truck with a larger engine takes a lot longer to cool than a silver car with a smaller engine. Similar interior space as the truck is a four door.

    The Infiniti has a kick ass AC unit though, which may explain why it only gets good gas mileage and not great gas mileage. Though it is tuned to outperform the best performing economy vehicles like the Civic or Corolla. I had previously owned a Civic though, and the G20T/Primera is head and shoulders above a standard economy car.
     
  18. mactastic macrumors 68040

    mactastic

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    Apr 24, 2003
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    Colly-fornia
    #18
    Not only has AC gotten more efficient, but the drag coefficient has as well. On a highly aerodynamic vehicle I'd think any little increase in drag would be a lot more significant than on something like a truck or an SUV. I mean, if I leave the tailgate on my truck up that's a huge amount of drag. But that's normal for that vehicle. I wonder what it would do to the gas mileage if my wife drove her Civic with the trunk open all the time.
     

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