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Old 01-21-2011, 04:27 PM   #1 (permalink)
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coasting a GM in neutral not economical?

...this Car-and-Driver article has some rather interesting information about GM products that eco-drivers might want to know:

When Coasting to a Stop, do Not Shift Into Neutral - Feature - Car and Driver

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Old 01-21-2011, 05:14 PM   #2 (permalink)
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I think the important thing is coasting to a stop, stop being the crucial factor.
If you have to stop then coasting in gear is better if you can slow down at the proper rate of deceleration to stop in time.

That assumes you have placed yourself in a position to have to stop in the first place, at a rate of deceleration that would require engine braking.

My Altima gets 60 MPG (indicated) coasting at 16 MPH with the engine warm (idling) and no accessories running.

It IS more efficient to use DFCO if you have to stop completely. It is much more efficient to anticipate the traffic signal and coast for several tenths of a mile waiting for the light to change so you do not have to stop completely.

While DFCO gets me infinite mileage, coasting at 64 MPH with the engine running gets me 240 MPG during the coast, and that would last a whole lot longer in neutral than in gear using DFCO.

20 miles down the Interstate today from a dead cold engine got me 41.3 MPG indicated, which is actually about 39, in a car with a rating of 31. I can coast down the downslopes of the overpasses and lose only 5 MPH. Those overpasses are about the steepest grades around here unless you head west into the Piedmont region of eastern Virginia. That is in heavy traffic with a PSL of 65 MPH.

It does require significant forward distance anticipation to use coasting instead of engine engaged DFCO, but both are tools that can make a big difference in mileage.

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Old 01-22-2011, 01:41 AM   #3 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...this Car-and-Driver article has some rather interesting information about GM products that eco-drivers might want to know:

When Coasting to a Stop, do Not Shift Into Neutral - Feature - Car and Driver
An article found in Car & Driver magazine is hardly the great authority or final word on the subject.

Personally, I don't believe this. I find a considerable increase in FE shown in my '09 Aveo (which is a GM product) as indicated by a ScanGauge when utilizing coasting in neutral to a stop. Also, as expected, the car consumes slightly less fuel while stopped at idle while in neutral rather than in drive. To me the only important question is whether there is any possibility of it being detrimental to the transmission. I've been told by a transmission shop that it isn't. Essentially it's the same effect as towing a car with it in neutral. This applies if you actually coast to a stop or a near stop where the tranny can go into first gear. It may be detrimental if you are shifting it back into gear at upwards of 20-30 MPH or more, however.

Bear in mind that for legalistic reasons you will not find this magazine or any other media source willing to endorse neutral coasting in their publication. They are not about to stick their necks out and take heat from law enforcement agencies for recommending it. It is technically not legal to do it, owing to the prevailing legalistic argument that you don't have the same amount of "control" over your vehicle as you do when the transmission is in drive.
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Old 01-22-2011, 02:18 AM   #4 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...this Car-and-Driver article has some rather interesting information about GM products that eco-drivers might want to know:

When Coasting to a Stop, do Not Shift Into Neutral - Feature - Car and Driver
The article just talks about DFCO, which is not unique to GM vehicles. Makes perfect sense; if you've got to slow down anyway, might as well slow down without using any fuel at all. That could be done by shutting off the engine, or by leaving the car in gear so that the engine is forced to rev higher than the fuel cutoff point. If you take it out of gear with the engine running the engine *must* use fuel in order to keep running. Not much, but more than none.

Old Mechanic described it perfectly:
Quote:
While DFCO gets me infinite mileage, coasting at 64 MPH with the engine running gets me 240 MPG during the coast, and that would last a whole lot longer in neutral than in gear using DFCO.
When I'm sure I've got to stop anyway, I will leave the car in gear. When I don't expect I've got to stop, I'll take it out... even though it uses a small amount of fuel, I'll get a lot farther.
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Old 01-22-2011, 02:42 PM   #5 (permalink)
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...Tom Reid is a GM powertrain spokesperson and, I believe, can be taken at his word.

...if nothing else, the article informs people of GM's stance on this driving technique and its' pros & cons, as viewed from their view point.

...my thought: "...information, like dynamite, is only a tool; it's what you *do* with them, that makes them useful or dangerous" (ha,ha).
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Old 01-22-2011, 05:38 PM   #6 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...Tom Reid is a GM powertrain spokesperson and, I believe, can be taken at his word.

...if nothing else, the article informs people of GM's stance on this driving technique and its' pros & cons, as viewed from their view point.

...my thought: "...information, like dynamite, is only a tool; it's what you *do* with them, that makes them useful or dangerous" (ha,ha).
It depends on how you define "coasting". To them, apparently what they are calling coasting is simply not having your foot on the gas while in gear (on an A/T, being in "drive").

To me, coasting is moving along without having the car in gear. Case in point: if you were driving a manual and you were descending a hill while in gear, you wouldn't call it "coasting"; in fact it would be providing engine braking. You'd call it coasting if you didn't have it in gear.
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Old 01-22-2011, 06:42 PM   #7 (permalink)
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Am I the only one that finds it ironic that the article seems to be talking about GM Automatics and yet the photo alongside it shows a BMW Manualgear lever ?
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Old 01-22-2011, 09:41 PM   #8 (permalink)
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...a "stock" C&D picture maybe?
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Old 01-23-2011, 06:45 AM   #9 (permalink)
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And they don't have an Auto one ?
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Old 01-23-2011, 08:07 AM   #10 (permalink)
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Quote:
“Shifting into neutral in an automatic will cancel fuel cutoff. Thus, it is better to remain in gear and let the drive wheels pull the engine airflow down to where fuel cutoff can be enabled or where fuel flow is minimized.”
Old Mechanic pegged it, I think. The C&D author seems to be missing the point about the difference between coasting to a stop and coasting for distance.

And yes, it is very strange how they show a manual shift SIX speed (by the look of it?) -- the layout people are not thinking?

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