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Old 10-05-2009, 11:15 PM   #1
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When coasting, do not shift to neutral for better mpg?

In a recent issue of Car and Driver, they reviewed gas mileage myths and said something I have a hard time believing:

When Coasting to a Stop, do Not Shift Into Neutral
"We went in search of an expert opinion on this one. According to Paul Williamsen, the product education manager at Toyota, “All contemporary Toyota and Lexus vehicles (and every other car built since the 1990s that I’ve looked at) can detect the condition when engine revs are higher than idle with a fully closed throttle: Under these conditions, all current to the fuel injectors is stopped, and no fuel is injected.” That means if your foot is off the gas while the car’s in gear, you’re not using any fuel.
Tom Read, GM’s powertrain spokesman, agrees: “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."

from: http://www.caranddriver.com/features/08 ... pg-feature

Does anyone believe this is true for our vans? My scanguage might show an instantaneous mpg of ~35 coasting down a long straight hill, but if I shift to neutral it jumps up to 99. Seems the Scanguage thinks I am using less fuel in neutral. Going through the mountains, I am sometimes tempted to use this technique during the descent - especially since otherwise my cruise control abruptly/violently downshifts as my coasting speed increases. Does anyone know if this "Car and Driver" advice is correct?

Thx,
-JR
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Old 10-06-2009, 01:10 AM   #2
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Re: When coasting, do not shift to neutral for better mpg?

My V10 does not do that, nor does my Camry. Both will use less fuel in neutral.

My Mom's 3.0 Jaguar S-type however, uses the least fuel in gear.
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Old 10-06-2009, 08:30 AM   #3
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Re: When coasting, do not shift to neutral for better mpg?

Not that it makes any difference to anyone but coasting in neutral is illegal, at least in California:

http://www.dmv.ca.gov/pubs/vctop/d11/vc21710.htm

But to address the issue, yes, I believe it...why wouldn't someone believe it since it is coming from experts? Coming down Donner Summit a couple of weeks ago, in gear, my Scanguage peaked at the maximum reading. Besides, I'd trust experts more than instantaneous readings from the Scanguage. More important, I'd rather trade fuel mileage for brake wear.
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Old 10-06-2009, 09:47 AM   #4
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Re: When coasting, do not shift to neutral for better mpg?

You must have a gas engine. My 6.0 diesel shows 9999mpg anytime I take my foot off the throttle when moving more than 30mph. The engine CPU shuts off fuel flow during coasting. Obviously it uses some fuel when idling.

I would avoid shifting into neutral for a couple of reasons:

- You need to use the brakes more to slow down
- You can't accelerate quickly should the need arise
- More wear and tear on the shifting mechanism

But, it is your van and you make your own choices. As long as you are happy with your choices, fine with me.

My pickup has a manual transmission. It uses more fuel in gear than it does idling when slowing down. I think it has to keep sending fuel in to keep the catalytic converter at full heat. Otherwise the engine is just pumping air into the exhaust system.

Mike
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Old 10-06-2009, 10:07 AM   #5
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Re: When coasting, do not shift to neutral for better mpg?

Quote:
Originally Posted by Ford_6L_E350
You must have a gas engine.
Yes, I have the v10 gas engine ('06)


Quote:
Originally Posted by Ford_6L_E350
I would avoid shifting into neutral for a couple of reasons:

- You need to use the brakes more to slow down
- You can't accelerate quickly should the need arise
- More wear and tear on the shifting mechanism
I only coast in N when the hill is long and straight. If it is curvy or steep, I keep it in gear, and maybe downshift to save my brakes.

I can't imagine a need to accelerate quickly in these situations - I'm already coasting at highway speed.

I do worry about the shifting mechanism, so I only do it when the hill is very long. But best of all would be to discover this is entirely unnecessary because of the injector shutoff. Perhaps the scanguage is calculating mpg from rpms and doesn't detect the injector shutoff in my vehicle? I'd love to know somehow?
-JR
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Old 10-06-2009, 10:53 AM   #6
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Re: When coasting, do not shift to neutral for better mpg?

ScanGauge gets the injector on times from the CPU to calculate fuel use. It's pretty accurate. However, if you look at consumption instead of MPGs, the difference between 35mpg and 99mpg is really miniscule.

At 35 mpg, you are using 3.65 ounces of fuel/mile

At 99 mpg, you are using 1.30 ounces of fuel/mile

To save 1 gallon you have to shift into neutral instead of coasting in gear, for 55 miles. Or, you save about $0.05-.06 per mile. That's a long downhill to save a nickel. You have to decide if it is worth it for you.

Mike
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Old 10-06-2009, 11:01 AM   #7
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Re: When coasting, do not shift to neutral for better mpg?

There was a discussion about this last year: Diesels and Engine Braking: Ken vs. His Transmission.

I think the gist is: for gas engines, it's unclear what's happening but it's not unlikely the Ford gas engine cuts fuel off completely when engine braking. The Scangauge computes MPG indirectly from other factors -- so it probably wouldn't realize that fuel has been cut off.

If this is the case, it's unclear whether you're using more fuel long term:
  • coasting down a hill in Neutral with gas being used to turn the disconnected engine at idle RPMs[/*:m:2qa7vey6]
  • coasting down a hill in Drive with gas cut off completely, but with slight engine braking because the motion of the van is turning the engine at low RPMs[/*:m:2qa7vey6]

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Old 10-06-2009, 11:04 AM   #8
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Re: When coasting, do not shift to neutral for better mpg?

Now the real efficiency (and fun) begins when
  • coasting down a hill in neutral with the ignition turned off[/*:m:dtbn3321]



It doesn't really warm my heart that the V10 uses as much fuel idling as my Prius does driving around on average.
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Old 10-06-2009, 02:55 PM   #9
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Re: When coasting, do not shift to neutral for better mpg?

Quote:
Originally Posted by geoffff
There was a discussion about this last year: Diesels and Engine Braking: Ken vs. His Transmission.

-- Geoff
Now we know who has a memory that goes back further than breakfast. But, really that was over a year ago. I had three posts in it and no memory of the thread. The benefits of old age.

Mike
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