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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 Active 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).
     

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