In Reply to: 2-3 mpg for AC on? Source?...., Mike Lynch [Profile/Gallery] , Sat, 16 Aug 2003 16:04:10

As far as I've heard or understood from reading Car and Driver, et.al. for 35 years, government MPG testing is definitely NOT done with the A/C on. Interesting thought, though.

I suppose if you live in the south, there would be some precedent for doing that. In Wisconsin, my A/C is on about one month a year. (Other than the defrost A/C heat combination, which is always a good idea to leave on.) The rest of the time, I crack the right side windows and lift the sunroof. MPG seems to stay the same.

Wide open windows at 75 MPH increase the drag coeficient, especially on a car as aero as the Saab 9-5. The laminar flow is disrupted into "turbulence" along the back window, creating a larger vacuum than would otherwise be there, and actually dragging large volumes of air along with the car instead of letting it slip on by.

I'm not sure if "cracking the window" does this as well, or if the sunroof disrupts flow in the same fashion, or acutally helps fill the void.

In Wisconsin, with long flat straight freeways, and minimal slowdowns, it is easy to simply set the MPG gauge and do continual readings as you cycle the A/C off and on in perhaps 5 mile increments.

Every car I have ever owned, has seen appx an 8-10% MPG penalty with the A/C on. (at 75 mph)

Opening the windows all the way at that speed shows an even worse MPG penalty, but who can stand to test it???

I had the worst A/C penalty on a 89 Taurus. The 9-5 isn't that bad, comparatively. Of course, the faster you go, the more sense it makes to use the A/C instead of opening the windows.

On the other hand, if you are in stop and go traffic, the A/C penalty is far worse than opening a window or four.

Your results may vary.

IMHO, Craig



posted by 206.191.194...

• Question on startup of 9-5, Saabnormal, Fri, 15 Aug 2003 19:45:25
  • I'm with your wife....and Ian...., Mike Lynch , Sat, 16 Aug 2003 16:00:30
  • Re: Question on startup of 9-5, boogiemouse, Sat, 16 Aug 2003 10:31:24
    • 2-3 mpg for AC on? Source?...., Mike Lynch , Sat, 16 Aug 2003 16:04:10
      • Re: 2-3 mpg for AC on? Source?...., boogiemouse, Sat, 16 Aug 2003 21:50:37 <-- Viewing This Message
  • I asked same question of dealer mechanic.., Al S, Fri, 15 Aug 2003 22:59:40
  • Let's Make a Deal, Ian Glenday , Fri, 15 Aug 2003 20:56:50
    • I don't think that it's the A/C compressor, Aero'ed, Sat, 16 Aug 2003 06:03:48
      • Other than the Blower Motor, the Compressor is, Ian Glenday , Sat, 16 Aug 2003 10:17:53
  • Re: Question on startup of 9-5, Ed NC, Fri, 15 Aug 2003 19:51:28
    • I second Ed. . . ., JPW, Fri, 15 Aug 2003 21:00:47
      • Re: I third Ed. . . ., JCarpio, Sat, 16 Aug 2003 07:33:38

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