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Old 07-05-2010, 01:58 PM
Shatto Shatto is offline
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Default COASTING & A/C vs WINDOWS

July 2010 Popular Mechanics, "The Truth About Energy" issue. Page 78.

Myth: Coasting to a stop saves gas.

Truth: A car actually consumes more fuel coasting in neutral than it does in gear. That's because absent any throttle input, an engaged engine reduces the flow of fuel to the injectors.

My Guess: Shifting to neutral while stopped takes a load off the system and saves fuel.



Myth: Driving with the A/C off and windows saves fuel.

Truth: It takes 2 to 8 hp to run the A/C, so when driving at city speeds, turning it off does save energy. But on a highway, open windows can significantly increase drag.

My Guess: I usually open the driver window and crack the passenger window a few inches at speeds below 45 MPH because it's less stuffy and the wind noise is acceptable.
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Old 07-05-2010, 06:04 PM
John Kunkel John Kunkel is offline
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Opening a window on a 90° day lets 90° air enter the car regardless of speed, even a poorly performing A/C can better that.
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Old 07-05-2010, 06:55 PM
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bulldog426 bulldog426 is offline
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Quote:
Originally Posted by John Kunkel View Post
Opening a window on a 90° day lets 90° air enter the car regardless of speed, even a poorly performing A/C can better that.
it's moving air though, just like a ceiling fan in your house or a box fan in your shop, it's the same degees of air, but it's moving air... i live in the south and on a 98 degree day with a heat index of 105 i can drive with my windows down while going about 40mph and stay cool, plus i work outside, and that's the reason i do it is to keep from getting sick from jumping in and out of hot air to cool air, plus i've noticed i use less fuel in my work truck.. i saw it on mythbusters and they used two of the same exact vehicles set them on the exact same speed and went to an asphalt oval racetrack, ran one with windows down, and one with ac, the one with windows down ran alot longer without running out of gas
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Old 07-05-2010, 10:10 PM
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ehostler ehostler is offline
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On an old car with the V-twin compressors, windows down is a better choice for fuel economy. On a newer vehicles with the newer design compressors, there is no difference between open windows or AC.
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Old 07-06-2010, 04:06 PM
John Kunkel John Kunkel is offline
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Quote:
Originally Posted by bulldog426 View Post
i live in the south and on a 98 degree day with a heat index of 105 i can drive with my windows down while going about 40mph and stay cool,
Stay cool compared to what? Comfort is a personal thing, I definitely wouldn't feel "cool" under the circumstances you outline.
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Old 07-06-2010, 08:30 PM
dodger1 dodger1 is offline
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Quote:
Originally Posted by bulldog426 View Post
i saw it on mythbusters and they used two of the same exact vehicles set them on the exact same speed and went to an asphalt oval racetrack, ran one with windows down, and one with ac, the one with windows down ran alot longer without running out of gas
What speed was the test done at? What kind of vehicles? (low-drag small car or high-drag pick-up?) I can assure you that when the temps here are over 30C, (85F) and we need to go anywhere out of town, I'd rather be in our Jimmy with AC than the Dodge with open windows, not to mention the lack of noise and road dust. But that's my personal tolerance, if I lived on the Gulf Coast I'd likely tolerate heat much better than I do.
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Old 07-08-2010, 03:01 AM
fat 360 fat 360 is offline
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Gotta have my say on this one...My daily driver is a 95 cherokee,when I do 60mph into a strong headwind with the drivers window down,then I wind it up,the car feels like it's got less throttle effort to maintain the same mph.
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Old 07-09-2010, 11:18 AM
Cudadrag Cudadrag is offline
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The coasting thing might make sense. Its why manual trans cars get a little better mileage than auto. (among other things.) I have seen shows that tried to compare the Window vs air conditioning. And what was determined is that it is indeterminate. Allot depended on which vehicle is tested. It appears that vehicles like trucks and other poorer aerodynamic vehicles did better with the window. New aerodynamic conscience cars did better with air. But even with this general rule (Like most everything else in life) doesn't apply 100% of the time.
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