COASTING -- & -- A/C vs WINDOWS - Fuelly Forums

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Old 07-05-2010, 10:39 AM   #1
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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, 10:52 AM   #2
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As far as the first one is concerned, Deceleration Fuel Cut Off isn't quite as guaranteed as they imply but it's still usually much better than coasting to a stop.

For your idea of shifting to neutral while stopped, it didn't make a measurable difference for me and I got tired of waiting for Drive to engage when it was time to go. Also, there's the concern about extra wear on the 1st gear clutch (or whichever engages when you stick it in D from N); it would be small but still extra. It may work better for other people.
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Old 07-05-2010, 10:57 AM   #3
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I didn't experience a noticeable difference in mileage when I tried N at stops, so I stopped it as well.
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Old 07-05-2010, 11:49 AM   #4
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My wife's Prelude SI vibrates like crazy at idle so I shift into neutral to save my teeth.
It is the first generation automatic and, I'm told it is a modified manual, or something like that. In any case it was so bad the original lasted only 30,000 miles before all the clutches wore down to metal.

I shift the Tundra out of gear at stop for the same reason I did the Dakota. But I've done it so long I can't recall why I began.
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Old 07-05-2010, 12:17 PM   #5
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If you're talking about auto trans, then I agree. A manual can be shifted to neutral with the engine off to save the most gas.

I also manage my window use according to vehicle speed; fully down at stops, half up around town, fully up with fans on for freeway use.
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Old 07-05-2010, 12:58 PM   #6
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Quote:
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My wife's Prelude SI vibrates like crazy at idle so I shift into neutral to save my teeth.
sounds like you(she) may need engine and tranny mounts.
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Old 07-06-2010, 10:04 AM   #7
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sounds like you(she) may need engine and tranny mounts.
I know my Stratus is probably vibrating like crazy for this. The steering wheel shimmers all of the time while down any kind of idling.

...but I'm also got a weird timing belt issue that I can't afford to fix right now. Putting all of the money into the Honda VX that is related to car PM work. Save for regular maintenance.
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Old 07-06-2010, 01:04 PM   #8
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Quote:
Originally Posted by shatto View Post
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.
I find this to be far from accurate. For example at 20mph I can coast to a red light about 75yards away. To do it in DFCO mode I would need to down shift to 2nd, which would make me slow down quickly, so I wouldn't let off the throttler until I was 25yards away. So the comparison would look something like 30sec of idling vs. 15s @ 1800rpm +7s of DFCO + 3sec at 1000RPM
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Old 07-06-2010, 01:49 PM   #9
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so those numbers are all greatly off, but the general comparison is about right
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Old 07-06-2010, 04:26 PM   #10
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With almost any fuel saving advice, one must experiment and measure to see if it really works for their combination of driver, route, traffic, and vehicle. DFCO is a great example of that, but most people don't have good information on when they're in DFCO and when not, nor do they try to find out because they assume that it's 100% dependable as implied by stuff like that article.
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