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Driving efficiency with windows or sunroof open?

Discussion in 'Model S: Driving Dynamics' started by Stimyg, Jun 15, 2013.

  1. Stimyg

    Stimyg Member

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    Anyone have any guesstimates as to how much driving with windows and/or the sunroof open hurts efficiency? And which hurts more, opening the windows or running the AC on low? Lets assume we're talking moderate speed highway driving, 65mph.
     
  2. cinergi

    cinergi Active Member

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    I'm finding it to be WAY worse to drive with all windows down and sunroof open than to use A/C. I'm starting to think (and started some "testing") that the open sunroof is the biggest contributor.
    I'll easily go over 400 whpm on the highway when everything's opened up.
     
  3. spleen

    spleen Supporting Member

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    The AC hit is pretty minimal on efficiency so I'm guessing the windows being open (which really screws up the aerodynamics of the car) is probably the bigger impact but admittedly I don't have any numbers to back that up.
     
  4. bollar

    bollar Disgruntled Member

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    Hypermilers (and Mythbusters, FWIW) say that windows down is less efficient than AC on.
     
  5. richkae

    richkae VIN587

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    It wouldn't surprise me if a vehicle like a van - that is a brick pushing through the air - wouldn't notice much difference between windows up or down, but the Model S probably has the biggest difference between windows up and down of any car on the road because it starts out as the most aerodynamic.
    Therefore if AC is superior to windows down for any other car, I would bet AC is superior than windows down for the Model S.
     
  6. Stimyg

    Stimyg Member

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    That's what I figured. Any guesstimates as to what % efficiency you're losing with, say, driver+passenger window down? And sunroof?
     
  7. yobigd20

    yobigd20 Well-Known Member

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    I regularly drive 200 miles daily on highway @ cruise speeds between 70-90mph. I usually average 370Wh/mi. One day, I drove with windows down most of the way and the sunroof cracked and I averaged about 510Wh/mi for that run. It's definitely WAAAYY more inefficient. But that really goes without saying. It really kills gas mileage in ICE cars too. It just plain ol aerodynamics physics - more air resistance makes the car need to push harder to keep the same speeds. I'd say it's probably about 25-40% more inefficient. It's pretty significant. Rain also makes it worse. Not as much as open windows though.

    Here's a study GM did once. http://www.floridastategasprices.com/posts/Difference-between-efficient-and-inefficient-driving-could-be-1-200-yearly/1715-496835-1032.aspx
    in their tests of efficient vs inefficient, the 'efficient' driver got 575 miles on a tank, while the inefficient driver got 250.

    http://www.bankrate.com/finance/auto/will-rolling-down-windows-save-fuel-or-not.aspx
    So basically my results (37% more inefficient with windows down at fast highway speeds) confirm both GMs study and Consumer Report's study.
     
  8. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    It depends on speed. But if you hear buffeting, you're losing a lot. However, I've found that having the windows open just a small amount (2-3 cm) doesn't effect economy enough to measure. The pano roof makes too much noise when open (even in vent position) to ever use while driving. (Using vent position while parked it's great for keeping the car from heating up too much.)

    The a/c doesn't have much of a hit. The most economical way to use it, other than pre-cooling while charging, is to start a a fairly high temperature (just a bit lower than ambient) and then lower the temperature as you're driving. The auto fan should be at either 6 or 7 at any given time (one or two blades on the display).
     
  9. melindav

    melindav ☰ reserved

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    Do you never drive with your windows down, or at least cracked open? On a 71 day, I'll always have the windows down over AC
     
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  10. ccutrer

    ccutrer Member

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    It's typically more energy efficient to drive with your windows closed, and the A/C on. Wind resistance is a serious drag.
     
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  11. kingjamez

    kingjamez Member

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  12. Runt8

    Runt8 Member

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  13. Twiglett

    Twiglett Single pedal driver

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    Aero has more effect at high speeds, around town it will have almost no effect.
     
  14. ccutrer

    ccutrer Member

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    I'd like to see a study with EVs. In that study, the lower the fuel efficiency (the SUV), the more pronounced the losses of using A/C are. EVs are totally different beasts, though. Both they're more sensitive to wind drag (you don't notice it as much with ICE because they're so inefficient in the first place), and (at least in my Model X), the A/C is a pure electric highly efficient scroll compressor.
     
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  15. youlikeadajuice

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    There's a thread here...basically, Tesla is definitely more efficient with AC on vs. windows open.
     
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  16. wwu123

    wwu123 Member

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    I think the difference may lie with whether the car has a belt-driven compressor, or an electric compressor like the Tesla. My understanding of most belt/pulley-driven air conditioning compressors is that when on, they are basically full-on maximum all the time. If you set the temperature warmerr in the controls, the vents just dump most of the cooling by mixing in warmer ambient air. So awfully inefficient except on the hottest of days.

    The electric compressor/heat pump will only need to be on as much as you need actually cooling (or heating).

    At least I think I read that years ago.
     
  17. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    As Cinergi says: Driving with the windows and sunroof fully open is worse than the A/C. However, having the windows on one side only of the car open 20-30 mm is better than having the A/C on. The way I do it is up to about 30 (depending upon humidity) I have the windows open a bit. Over 30, it's A/C time.
     
  18. Zetopan

    Zetopan Member

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    The referenced study was for an ICE, which commonly has about one third of the efficiency of an induction run electric motor. It was also published in 2004 with the test vehicles being a "full sized" sedan (4.6L V8 engine) and a "full sized" SUV (8.1L V8), with both being GM products, neither of which (especially the SUV) being very likely to have aerodynamic characteristics that even approach a Tesla.

    I do not know any actual vehicle technical details since that "technical" paper is wonderfully absent any actual technical details, and similarly for the air conditioning being used. I am not aware of GM's "state of the art" in AC in 2004, but many years before that I had attended a GM technical seminar that describe the air condition system in a top of the line GM Cadillac. It had awful truly efficiency even for that time. The AC ran continuously and external hot air was mixed to produce the desired passenger compartment temperature. At that time, mileage for that specific vehicle was not even considered an important consideration for GM engineering. Presumably later designs were done in a far more conscious manner. The bottom line here being that the referenced SAE paper is wonderfully inapplicable for any Tesla AC vs windows down efficiency comparisons.

    Tesla owners can trivially perform the required experiments and report them here since every Tesla provides real time energy consumption feedback to the driver. Use the cruse control to set a constant speed for a given fixed stretch of road. Record the ambient and internal temperatures and energy consumption when traversing that exact same stretch of road (unimpeded it all cases). The test conditions should include having the AC on well before reaching that section of road (to let it stabilize) and with the AC off and windows both up and down, for a total of three test conditions. Also record Tesla model and year, the elevation, wheel size and tire model and pressure since those influence the drag and rolling resistance. The plotted data should easily provide for a higher quality of results than the referenced SAE "technical" paper, and would obviously be far more accurate and specific for the Tesla models tested.
     
  19. chibi_kurochan

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    on a side note, does anybody else experiencing super load cabin noise / resonance whenever you have one side of the windows down on the model s? not exactly sure how to describe it... and it typically starts once you go above 30-40 mph.. we have 2017 refreshed MS.
     
  20. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    I believe you're referring to buffeting. This happens with every aerodynamic car and sometimes non-aerodynamic cars. It's caused by the laminar airflow being disturbed, and happens when you open the windows more than some amount (likely varies somewhat by individual car).

    In the Model S, some folks have also had buffeting when the hatch stoppers are not correctly adjusted.
     
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