Coast in-neutral vs in-gear: I've been challenged to show the numbers

[reb01501]

Originally Posted by ecomodded

Coast down a secluded hill from a set starting point, till you reach a stop,mark the spot. Repeat it in neutral , repeat it all again. Then crunch the numbers.

That won't prove anything. That's essentially what the writer of the PM article that started this discussion did. What's needed is a net usage result as gealii posted. What we're trying to prove is that the gas lost while coasting in neutral and regaining the momentum lost during the coast is less than the gas that would be lost trying to regain the momentum lost after coating in gear.

There is no question that coasting in gear loses less gas DURING the coast than coasting in neutral ...

[ecomodded]

Originally Posted by RiceCake

In neutral is essentially guaranteed to coast further but proves nothing. The real test would be to see if the extra fuel needed to idle the engine while in neutral is /less/ then the fuel saved by driving the engine with the transmission in gear.

It will actually prove more than you think, i'll elaborate.

When in gear the motor / transmission become a brake if gas is not applied. Driving around with slight engine braking will lower your coast speed and length, meaning you will need to apply the throttle sooner then if you had it in neutral.

I have a sneaking suspicion that a car while in neutral and idling at 800 rpm is going to use less gas then a car engine braking at 3000 or more rpm.

these are the reasons i coast in neutral.

[reb01501]

Originally Posted by ecomodded

It will actually prove more than you think, i'll elaborate.

When in gear the motor / transmission become a brake if gas is not applied. Driving around with slight engine braking will lower your coast speed and length, meaning you will need to apply the throttle sooner then if you had it in neutral.

Well, duh! This is the point has been raised dozens of times in all these threads, without anyone documenting the savings - until gealii's post.

I have a sneaking suspicion that a car while in neutral and idling at 800 rpm is going to use less gas then a car engine braking at 3000 or more rpm.

Jeez! I thought we had established that with DFCO in effect, the engine is using ZERO gas, regardless of RPM, during in-gear coasting. It's the WHEELS making the engine's crankshaft spin at 3000 rpm, not the ignition of gas in the cylinders! The PM article does a good job of documenting this, all too well, unfortunately.

[ecomodded]

Originally Posted by UltArc

I hope I remember to test this next week! I am rather curious about this now. At first, for MY situation, I thought it almost for sure, but really breaking it down, I don't know. I normally bump start my engine, next time I won't, so it stays fair. I think in neutral there will be a lot more variables in play. In gear it is smoother, but hopefully we'll see soon.

The differences in coast lengths could be computed as engine braking losses, i suspect that coast lengths will be shortened by 30% if so then the gas will need to applied 30% sooner?? which would need to be measured and calculated.
I have my preconceptions so a test would be cool.
I think people could expect at least a 10% improvement by using neutral over Drive,
i will add i use neutral constantly in my driving.

[Whitey]

Hi, folks!

For what it's worth...

I've been driving my Honda for a bit more than a year now. A while back, I started using neutral for coasting while driving in town. Three tanks ago, I decided to start using neutral for coasting downhill on the highway.

I cruise at 57 MPH, and usually I'm able to maintain that speed during my neutral coasts. I had been unable to maintain that speed when "coasting" in gear and used a lot more gas. The evidence is in my fuel log (click the Honda badge in my signature). My last three tanks have been my three best tanks, averaging 47.01 MPG. The three tanks prior to those averaged 42.78 MPG.

-Doug "Whitey" Jackson

[roosterk0031]

On the way home tonight with Cobalt I did a quick ABA, long gentle hill, about 120 mpg 5the gear 55 mph, was slowing 1 mph per 2 seconds or so, so best guess within 10 mpg. Accellerated back to 55 mph, neutral coast 170 mpg, just steep enough to maintain speed so a pretty good number. Back to 5th gear 55 mph again pretty much 120 mpg. I haven't calibrated my SGII for this car yet, still on first tank, but comparing factory gauge to SGII it's in the ball park.

So roughly (160/120) 1/3 less fuel is used coasting with my car, plus it holds speed longer so coast could be started sooner so even more is really saved.

DFCO doesn't really seem effective with the little bit I've played with it in the Cobalt. Even with aggressive down shifting for braking, fuel consumption actually goes up initially, only after atleast 2-3 seconds often 5 or more if it does at all will it go into DFCO. With my Cobalt I need to resist using engine braking as it's wasteful.

[ecomodded]

Originally Posted by reb01501

Well, duh! This is the point has been raised dozens of times in all these threads, without anyone documenting the savings - until gealii's post.
Jeez! I thought we had established that with DFCO in effect, the engine is using ZERO gas, regardless of RPM, during in-gear coasting. It's the WHEELS making the engine's crankshaft spin at 3000 rpm, not the ignition of gas in the cylinders! The PM article does a good job of documenting this, all too well, unfortunately.

You have established absolutely nothing, besides being ill informed and slightly obnoxious,That i got.
Tell me how you suggest one measure for results?

[ecomodded]

Originally Posted by Whitey

Hi, folks!

For what it's worth...

I've been driving my Honda for a bit more than a year now. A while back, I started using neutral for coasting while driving in town. Three tanks ago, I decided to start using neutral for coasting downhill on the highway.

I cruise at 57 MPH, and usually I'm able to maintain that speed during my neutral coasts. I had been unable to maintain that speed when "coasting" in gear and used a lot more gas. The evidence is in my fuel log (click the Honda badge in my signature). My last three tanks have been my three best tanks, averaging 47.01 MPG. The three tanks prior to those averaged 42.78 MPG.

-Doug "Whitey" Jackson

That is a 10% improvement, Sweet !

[ecomodded]

Originally Posted by roosterk0031

On the way home tonight with Cobalt I did a quick ABA, long gentle hill, about 120 mpg 5the gear 55 mph, was slowing 1 mph per 2 seconds or so, so best guess within 10 mpg. Accellerated back to 55 mph, neutral coast 170 mpg, just steep enough to maintain speed so a pretty good number. Back to 5th gear 55 mph again pretty much 120 mpg. I haven't calibrated my SGII for this car yet, still on first tank, but comparing factory gauge to SGII it's in the ball park.

So roughly (160/120) 1/3 less fuel is used coasting with my car, plus it holds speed longer so coast could be started sooner so even more is really saved.

DFCO doesn't really seem effective with the little bit I've played with it in the Cobalt. Even with aggressive down shifting for braking, fuel consumption actually goes up initially, only after atleast 2-3 seconds often 5 or more if it does at all will it go into DFCO. With my Cobalt I need to resist using engine braking as it's wasteful.

The engine is indeed running when coasting in gear, OBVIOUSLY, as the car is already started!!!! some people have neglected to notice that, somehow,right reb0151

Those were fast tests from you two guys, they confirm what i posted, thanks for the favorable results :-)

[ecomodded]

here is that botched article from PM online.
All the same tomorrow i am going to test my car by coasting with the ignition off in gear then switch on the ignition and listen/feel for any change,like wise by testing with the ignition on in gear then switching it off.
Bust this myth, one way or the other.
I think the "Scangauge" reads injector pulse durations and has a function to read them,it would be useful to bust this Myth.
if anyone has one, take some readings while coasting in gear, engine on, its somewhere in the Scangauges menu.
No need to test with engine off as it will most definitely not be injecting any fuel.


the Article, finally !


Tactic No. 6: When Coasting Downhill, Leave the Car in Gear

There are those who refuse to be shaken from the practice of coasting downhill in neutral to save gas. This is a bad idea no matter how you look at it. Let's set aside fuel economy for a moment. Coasting downhill in neutral is illegal in most states. And it's dangerous in all states. In neutral, you have no way to accelerate to avoid a hazard, and if the engine stalls, you have no power steering or vacuum boost for the brakes. If the hill is steep enough to call for hitting the brakes to keep you from gaining speed, they're more likely to overheat—and overheated brakes lose effectiveness until they cool off. They'll probably do that right around the time the police show up to take the accident report.

Here's the surprise: There's no tradeoff between safety and fuel economy in this case. Leaving the car in gear while coasting downhill actually is more efficient. Why?

Most fuel-injected engines today use computer-controlled Deceleration Fuel Cut Off: When you lift your foot from the gas while leaving the car in gear, injectors shut off automatically, and the car's rotating tires—which are connected to the engine via the transmission—keep the engine turning and the accessories running. So, the engine consumes no fuel at all while the vehicle is coasting downhill.

In contrast, the fuel-consumption rate for an engine idling in neutral falls between 0.2 and 0.4 gallons per hour (gph). Splitting the difference and using 0.3 gph for our example, idling in neutral down a ½-mile-long hill consumes fuel for 30 seconds, for a total of about 0.32 ounces of gas. Popping the car into neutral actually wastes gas.

This may seem counterintuitive, but that's what data are for—replacing good guesses with solid facts. Watch the data, and over time the savings will take care of itself.

Read more: Driving Tips to Save Gas - Memorial Day Weekend - Popular Mechanics

Driving Tips to Save Gas - Memorial Day Weekend - Popular Mechanics