Better MPG coasting in Neutral or in Gear ?

[The motor guy]

I didn't read through all these posts but once your foot is off the throttle it should be putting in the same amount of fuel as it would if it were idling. The only thing I see as far as fuel savings is that in neutral their is no drag from the drive line so you can coast much farther.

 

[mechtech2]

You can't coast in gear, only in neutral.
The engine acts like a brake when in gear.

 

[kemo]

Originally Posted By: mechtech2
You can't coast in gear, only in neutral.
The engine acts like a brake when in gear.


yes you can, you hold down the clutch.

and as per wiki...

Quote:
When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required

 

[ahoier]

Originally Posted By: JHZR2
Coasting in neutral to a stop is ok. In an AT, ONLY do so if you know you will definitely stop. Otherwise you can damage your trans.

Doing it from speed to a dead stop only is perfectly fine, and we have multiple >200k ATs that have never been rebuilt.
Yes, this is what I've heard as well. Coasting to a "stop sign" or a red light (that you know will be red by the time you intend to stop) is OK, but I've heard of excessive transmission wear switching from "N" to "D" while the wheels are moving, as the trans/clutches will have to "catch up" as your wheels are already turning or something like that anyways lol...

Originally Posted By: mechtech2
You can't coast in gear, only in neutral.
The engine acts like a brake when in gear.


From my limited "coasting" I've noticed if I take my foot off the pedal (in Drive) going down a bridge (we don't have too many hills around here), my car will not travel as far as it would, if I had my vehicle in "Neutral"....

So something tells me the engine itself "slows the car down" for some reason.....been like that with all 3 cars I've owned thus far, 08 Kia Optima, 07 Kia Rio, and now hte 88 Dodge Aries

 

[rg200amp]

Originally Posted By: dparm
Originally Posted By: Eddie
Not a safe thing to do for a few oz of fuel.



I was taught in driver's ed never to let the car roll in neutral. If you have to take evasive maneuvers the last thing you want to think about is getting it back into gear.

Also puts excess wear and tear on the clutch, syncros, etc. Saving $20 a year isn't worth the possibility of a $600+ clutch job.


Yea, like down shifting while your slowing threw all the gears is going to add life to your clutch!!!!!!!

I throw my stick shift in N while slowing.

I can throw it in gear quicker than most automatics out there on the road anyway. And unlike auto's, I say what gear it goes into. You hit the gas in an auto, it may throw you into 3rd at 30mph at full throttle.

I can throw it into 2nd and then third, get quicker response, in the same time.

 

[demarpaint]

Originally Posted By: kemo
Originally Posted By: mechtech2
You can't coast in gear, only in neutral.
The engine acts like a brake when in gear.


yes you can, you hold down the clutch.

and as per wiki...

Quote:
When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required


How is an engine running when the fuel is cut off being driven by the wheels? That means the fuel pump is off, and the engine has stalled, and would need to be restarted to go again? Go easy I'm trying to grasp this. I can see if an AT or MT is left in gear the drive train will turn the engine, but that means the electric fuel pump is off. Now lets say the vehicle is placed in N, the wheels are no longer driving the engine, and the engine is still running. You still have spark, and pistons moving up and down, in N or in gear? Confusing??????????????? So the engine would use less gas if left in gear, since the wheels are turning the engine and there is no fuel supply, vs being left in neutral and the engine running on fuel???????????

 

[rg200amp]

Originally Posted By: kemo
Originally Posted By: mechtech2
You can't coast in gear, only in neutral.
The engine acts like a brake when in gear.


yes you can, you hold down the clutch.

and as per wiki...

Quote:
When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required


You are not considered in "gear" while the clutch is pressed in. That is the same a Neutral.

 

[FL_Rob]

Ya ,because its still in gear .With an automatic just taking your foot off the gas dosent mean its in nuetral,and your not nuetral coasting like in a manual trans.The TCC disengages and you still get compression braking through the clutch packs(wear,but minimal unless you're barreling downhill) at some point relative to vehicle speed, the trans re-engages a different gear based on speed or just go to default first at a stop.I've got a fluid change coming up .To test if the pump is still pressurizing the circuit in Nuetral,I'll perform the cooler line flush using nuetral instaed of park to check this out for myself.I dont put the trans in nuetral often coming to a stop but there is a long very slight downhill that I sometimes glide down in N instead of D.And rev matching just like with a manual trans does work to minimize "shift shock".

 

[d00df00d]

Originally Posted By: demarpaint
How is an engine running when the fuel is cut off being driven by the wheels? That means the fuel pump is off, and the engine has stalled, and would need to be restarted to go again?

The engine is not stalled because it's still turning. The fuel pump will still be running and keeping the fuel rail pressurized. The only difference is that no fuel is being injected because combustion is not needed to keep the engine turning.

If you disengage the clutch, put the transmission in neutral, or simply slow down enough, the RPMs will drop. Once they drop below a certain point, the engine will resume fuel injection and ignition to keep the engine turning, just as it does at idle.

Make sense?

 

[demarpaint]

Originally Posted By: d00df00d
Originally Posted By: demarpaint
How is an engine running when the fuel is cut off being driven by the wheels? That means the fuel pump is off, and the engine has stalled, and would need to be restarted to go again?

The engine is not stalled because it's still turning. The fuel pump will still be running and keeping the fuel rail pressurized. The only difference is that no fuel is being injected because combustion is not needed to keep the engine turning.

If you disengage the clutch, put the transmission in neutral, or simply slow down enough, the RPMs will drop. Once they drop below a certain point, the engine will resume fuel injection and ignition to keep the engine turning, just as it does at idle.

Make sense?


Yes thanks, but.............. A few more questions if you don't mind. If a car is shut off in drive does it coast the same as it would if it was coasting in drive with the engine running? Or is there more drag when the engine is turned off? When an engine is turned off coasting in gear, power steering, power brakes, etc. are lost because the engine is off. Yet if a car is coasting in drive with the engine on, all accessories are working because they are getting power from the engine, and in order to get power from the engine it must be running. An engine can't run w/o fuel, and there lies my confusion in this matter.

 

[FL_Rob]

Are you refering to an auto or manual? ,they behave differently with the engine switched off... very differently.

 

[d00df00d]

Originally Posted By: demarpaint
If a car is shut off in drive does it coast the same as it would if it was coasting in drive with the engine running? Or is there more drag when the engine is turned off?

All else equal (torque converter lockup/clutch engagement etc.), there would be MUCH more drag with the engine off.


Originally Posted By: demarpaint
When an engine is turned off coasting in gear, power steering, power brakes, etc. are lost because the engine is off. Yet if a car is coasting in drive with the engine on, all accessories are working because they are getting power from the engine, and in order to get power from the engine it must be running. An engine can't run w/o fuel, and there lies my confusion in this matter.

Power steering assist is driven by the belts, as is the alternator which powers the electrical accessories. The belts will continue to move as long as the engine is turning.

Power brake assist usually comes from engine intake vacuum, which is in no short supply when you are coasting in-gear.

 

[FL_Rob]

The engine is still being metered fuel and still given spark even with the throttle closed ,just like idle.In fact that's the definition of idle.

 

[JHZR2]

Coasting in gear with no throttle, there is no fuel being injected, but there is a mechanical couple causingthe engine to spin. It is not a stall, there is enough mass inertia due to rolling along to keep the engine spinning though no fuel is being burned. Remember that all an engine really is, is a big air pump.

So closed throttle, over 1100 rpm or so and fuel shuts off and the engine just pumps air. It would be nice if engines had the smarts to either extend valve overlap or part-open the throttle plate so that coasting could retain a lot more inertia yet not have to do it in neutral burning fuel to get max distance... The issue is that closed throttle coasting induces huge flow resistance (due to the closed throttle and the compression that still occurs), which eats inertia... Thus the comment that it acts like a brake.

 

[Colt45ws]

I dont get how switching from N to D while moving would hurt anything. Really all you are doing is engaging clutches...
Which is what it is doing to change gears.

 

[eljefino]

Originally Posted By: demarpaint


Yes thanks, but.............. A few more questions if you don't mind. If a car is shut off in drive does it coast the same as it would if it was coasting in drive with the engine running? Or is there more drag when the engine is turned off? When an engine is turned off coasting in gear, power steering, power brakes, etc. are lost because the engine is off. Yet if a car is coasting in drive with the engine on, all accessories are working because they are getting power from the engine, and in order to get power from the engine it must be running. An engine can't run w/o fuel, and there lies my confusion in this matter.


No because "Drive" in most cases (sans fuzzy logic) is a "freewheeling" gear with no engine braking.

Try this, idle down a smallish hill in drive where the trans is in 1st or 2nd. Downshift to the manual version *of the same gear* and you'll feel some engine braking, as the transmission is set up to give braking *in the lower gears*. Or just start off on a flat surface in manual 1st and take your foot off the gas; she'll slow down faster than normal.

(Newer tranny computer logic can tell when you're going down a hill, provided you're off the gas, and will sometimes hold a gear or provide effort to not speed up on you. Touch the gas to speed up and they'll upshift and start freewheeling.)

 

[demarpaint]

Maybe it is the older tech cars I drive, and the newer Jeep I never took note.

Power steering and power brakes are lost when my 88 Ford and 93 Ford are coasting in gear and the engines are off. The 88 is a MT, the 93 an AT. Coasting with the engine on all is fine. I never let the 08 Jeep coast with the engine off, so I have no idea what would happen to brakes, and steering. My guess would be I'd lose the power assist? I thought the term fuel is shut off while coasting might mean, there is just enough fuel to maintain, heat or A/C, and all needed power while coasting. When completely stopped in gear, park, or neutral, an engine is burning fuel to keep running.

Thanks for the patience of those responding to me who are trying to help this older mind learn!

 

[Patman]

The hardcore hypermilers will coast in neutral but they also shut their engines completely off! They call it pulse and glide, they speed up to a speed higher than the speed limit, throw it in neutral and shut the engine off and then coast for a while and then restart. That's insane though!!

 

[Vikas]

You have to understand that UltraGauge "computes" fuel consumption. It is NOT reported to it. It does that based upon MAF reading and engine speed and engine load. When MAF is not available, it uses MAP reading and assumed volumetric efficiency. It does NOT know the fuel injection pulse width. If the engine is cutting out fuel during the engine braking and/or coasting, UltraGauge will NOT show it.

Coasting in "N" on an automatic is fully retarded, I can not emphasis that enough and I used the term "retarded" intentionally. Don't do that.

- Vikas

 

[eljefino]

Originally Posted By: demarpaint
Maybe it is the older tech cars I drive, and the newer Jeep I never took note.

Power steering and power brakes are lost when my 88 Ford and 93 Ford are coasting in gear and the engines are off. The 88 is a MT, the 93 an AT. Coasting with the engine on all is fine. I never let the 08 Jeep coast with the engine off, so I have no idea what would happen to brakes, and steering. My guess would be I'd lose the power assist? I thought the term fuel is shut off while coasting might mean, there is just enough fuel to maintain, heat or A/C, and all needed power while coasting. When completely stopped in gear, park, or neutral, an engine is burning fuel to keep running.

Thanks for the patience of those responding to me who are trying to help this older mind learn!


I don't think many are talking about manually switching off the key. Simply taking one's foot off the gas on most stick shift cars lets the car's inertia turn the engine over, which is sparking but not injecting fuel, and turning over the belt driven accessories, and making vacuum for the power brakes. Imagine some cunning automatics might be able to pull this off as well.