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Improving gas mileage... coasting in gear uses less gas than coasting in neutral?

psteng19

Diamond Member
Dec 9, 2000
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Well, my gas mileage again has plummeted from a consistent 43 mpg to about 35 mpg due to the winter season (mostly gas and warming up).
So being the anal guy I am about fuel efficiency, I'm looking for the best ways to improve fuel economy.

I've read in past threads that coasting in high gear uses less gas than coasting in neutral.
How is that so?
So coasting in 5th gear at 65 mpg with engine about ~2500 RPM is using less gas than coasting in neutral (idle) at 800 RPM?
I know idling uses next to no gas, so coasting at 2500 RPM in gear would have to use like no gas and only the current momentum to keep me going. So the injectors don't pump gas into the chamber when coasting in gear?

What about using cruise control? I see publications telling us to use cruise control to keep RPM's constant (assuming flat roads).
How is it different if we were to keep a constant RPM manually?
Probably not much, if any right?
 
Nov 7, 2000
16,404
3
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cruise control when i have used it kept my speed constant, not the rpm. meaning, going up a hill, its gonna increase rpm to maintain speed
 

DrPizza

Administrator Elite Member Goat Whisperer
Administrator
Mar 5, 2001
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www.slatebrookfarm.com
I'm curious to also hear an explanation why coasting in neutral uses more gas than coasting in high gear - I'd have thought it was the opposite:
Reason: coast downhill
Put car in neutral, turn off engine..
vs. Coast downhill, with car in gear, turn off engine

In the latter case, the engine slows the car. So, it seems to me that having the car in gear adds a frictional force to the car.

Now, with the engine on, it seems that 800rpm's will use less gas than 2500 rpm's, unless, as the OP said, no fuel is being injected when coasting at 2500 rpm's.

<--- engine n00b


edit: re: coasting in gear - since there's a frictional force, on a level surface, your car shouldn't be able to coast as far while in gear than it can when out of gear with the engine off... Try it: Get up to 55mph on a back road - leave car in gear and turn off engine, see how far you coast.
Try again: put car in neutral, turn off engine, coast. You'll go farther.
 

Mareg

Member
Jul 24, 2004
170
0
0
I'm also very interested in this topic.

Usually, when I have the possibility to shift in neutral at high speed I would do it (thinking I would save some gas). But I always have the impression that when I shift back in 5th, my gas indicator has gotten significantly lower.... to what I told myself this was just my immagination but now I might consider otherwise.

Someone as a coherent explication ?
 

spidey07

No Lifer
Aug 4, 2000
65,469
5
76
doesn't seem right, but I'll take a stab at it.

Modern computers in a car could be sending less gas to the motor while coasting in gear, possibly even less than idle. engine RPM and rate of fuel aren't necessarily closely related - its all about the throttle and ECU.
 

MikeMike

Lifer
Feb 6, 2000
45,885
66
91
From what ive seen, atleast on my vehicle, when i shift into neutral going down a hill etc, the engine RPM's are higher than when just coasting in gear, no clue why though, it just does it.

MIKE
 

Colt45

Lifer
Apr 18, 2001
19,721
0
0
if you take your foot off the gas in 5th, then engine will brake the car, compression and all..

so neutral will be more efficient. well. you'll go a further distance for the same intertia
 

conjur

No Lifer
Jun 7, 2001
58,686
2
0
Originally posted by: nourdmrolNMT1
From what ive seen, atleast on my vehicle, when i shift into neutral going down a hill etc, the engine RPM's are higher than when just coasting in gear, no clue why though, it just does it.

MIKE
Mine drop to about 800rpm but I don't have one of those fancy trip computers.
 

desy

Diamond Member
Jan 13, 2000
5,388
151
106
OK when your coasting down just cause the engine is higher RPM doesn't mean it will use more gas, other wise you'd be accelerating instead of engine breaking.
I think it might be cause you are engine breaking instead of using your breaks cause if you just click to neutral I hear it actually idle up a bit .
I wouldn't think it would be significant though stop and go you burn a lot more accelerating cause of slippage and leaving it idling are gonna impact it much more IMO
 

spidey07

No Lifer
Aug 4, 2000
65,469
5
76
Originally posted by: Colt45
if you take your foot off the gas in 5th, then engine will brake the car, compression and all..

so neutral will be more efficient. well. you'll go a further distance for the same intertia
you forgot about overdrive, so there wouldn't be any engine braking (in an automatic, manual is different ball game)
 

Eli

Super Moderator | Elite Member
Super Moderator
Oct 9, 1999
50,422
7
81
It's not really rocket science.

In general, the lower the RPMs, the less gas you will use. But idle is most certainly NOT the engines most efficient operating area..

The fuel usage graph of most engines looks a ( on its back. It starts out high at idle, and as RPMs increase to a high idle.. 1500.. maybe a bit higher... depending on the engine.. it decreases somewhat.. and then goes sky high as RPMs increase after that.

I don't know about using less gas coasting in gear than in neutral. I guess it depends on if the injectors stop firing when the engine RPM is above what it should be in relation to the throttle position. If they do, then it will use less gas. If they don't, and always inject as much fuel as the throttle position allows, then it shouldn't matter much.

Using the engine to brake probably does save fuel in another way overall, though. Everytime you hit the brakes, you are wasting energy. The less you use your brakes, the more fuel efficient you will be. Try to get into the habbit of coasting in gear whenever you can. Anticipate a stop and start slowing down before you normally would.

Obviously, you never want to push on the accelerator when it isn't truely necessary. If you drive... spirited.. that can be a major waste of gas. If you're the type of person that likes to speed up to a red light, that is also extremely inefficient.

Pretty much common sense stuff.... lol
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Okay, first of all, in a carbureted vehicle, coasting in gear uses more gas than coasting in neutral...period, end of story, no ifs, ands or buts.

However, on a fuel injected vehicle, this isn't necessarily the case. Most cars nowadays, if you coast for a few seconds, will completely shut off the fuel. So they aren't burning any gas, which is obviously less than the minute amount that it burns at idle.

You can usually feel it when the computer shuts the fuel off, too. But it you're mostly coasting, but tapping the gas, even just barely, then the fuel will never shut off.
So in that instance, flipping the shifter to neutral would be better.....then again, if you're occasionally tapping the gas, you probably need it to be in gear.

One more line of thought, though: If you're coasting in neutral, you're going to coast a lot farther as there's no drag from the engine or trans.
So even though the computer eventually shuts the fuel off while coasting in gear, overall, my money is on putting it in neutral being better most of the time....because you can't coast as far when you're in gear.

So you have the choice of burning very little gas at idle for a longer distance, or burning no gas for a much shorter time.
I'm talking total coasting distance here, such as if you coasted to a stop from highway speeds.
The real difference would be very miniscule, not even enough to worry about.
 

NutBucket

Lifer
Aug 30, 2000
26,463
277
126
Bingo, if you coast in gear then you will have your foot completely off the gas. The computer knows this and will shut off the injectors since the engine is turning over by gravity (well, the car moving via gravity). So, no gas consumption.
 
Oct 9, 1999
15,214
3
81
you never want to coast down the hill with the car in neutral.. and never with teh engine off.

you guys are n00bs... just drive a car like normal.. and forget about fuel efficiency, its the winter.. Plus your gettin 35mpg, which is good.. better than most ppl's car. Mine gives me 30..
 

psteng19

Diamond Member
Dec 9, 2000
5,953
0
0
Originally posted by: Pacfanweb
However, on a fuel injected vehicle, this isn't necessarily the case. Most cars nowadays, if you coast for a few seconds, will completely shut off the fuel. So they aren't burning any gas, which is obviously less than the minute amount that it burns at idle.

OK, so if you coast in gear and the computer shuts off the injectors, what keeps the engine turning?
The wheels (via momentum)?
Do the sparks fire?
 

andylawcc

Lifer
Mar 9, 2000
18,185
3
76
Top Gear mentioned something like that before: "Modern cars do save more gas when coasting in gear than coasting in neutral." but the definition of "modern" is rather vague.

If you're coasting in neutral, you're going to coast a lot farther as there's no drag from the engine or trans.
So even though the computer eventually shuts the fuel off while coasting in gear, overall, my money is on putting it in neutral being better most of the time....because you can't coast as far when you're in gear.
I think that's the answer to OP's question.
 
Jun 14, 2003
10,442
0
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if u coast in gear you use no petrol

if u coast in nuetral ur car uses petrol to keep the engin ticking over...not sure its that much of a deal like. also on top gear they said that they asked volvo how much extra fuel their daylight running lights used...they came back an said they couldnt measure it. but AUDI went ahead and said they would half it anyway

i also think going every where in high gear is bull, if you revs get too low your engine management is gonna start chucking more coal on the fire to keep the engine from stalling
 
Aug 16, 2001
22,529
4
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Originally posted by: psteng19
Well, my gas mileage again has plummeted from a consistent 43 mpg to about 35 mpg due to the winter season (mostly gas and warming up).
So being the anal guy I am about fuel efficiency, I'm looking for the best ways to improve fuel economy.

I've read in past threads that coasting in high gear uses less gas than coasting in neutral.
How is that so?
So coasting in 5th gear at 65 mpg with engine about ~2500 RPM is using less gas than coasting in neutral (idle) at 800 RPM?
I know idling uses next to no gas, so coasting at 2500 RPM in gear would have to use like no gas and only the current momentum to keep me going. So the injectors don't pump gas into the chamber when coasting in gear?

What about using cruise control? I see publications telling us to use cruise control to keep RPM's constant (assuming flat roads).
How is it different if we were to keep a constant RPM manually?
Probably not much, if any right?
Actually coasting in gear uses NO gas because the ECU on modern cars turns off fuel delivery.
 
Jun 14, 2003
10,442
0
0
Originally posted by: FrustratedUser
Originally posted by: psteng19
Well, my gas mileage again has plummeted from a consistent 43 mpg to about 35 mpg due to the winter season (mostly gas and warming up).
So being the anal guy I am about fuel efficiency, I'm looking for the best ways to improve fuel economy.

I've read in past threads that coasting in high gear uses less gas than coasting in neutral.
How is that so?
So coasting in 5th gear at 65 mpg with engine about ~2500 RPM is using less gas than coasting in neutral (idle) at 800 RPM?
I know idling uses next to no gas, so coasting at 2500 RPM in gear would have to use like no gas and only the current momentum to keep me going. So the injectors don't pump gas into the chamber when coasting in gear?

What about using cruise control? I see publications telling us to use cruise control to keep RPM's constant (assuming flat roads).
How is it different if we were to keep a constant RPM manually?
Probably not much, if any right?
Actually coasting in gear uses NO gas because the ECU on modern cars turns off fuel delivery.


correct
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: psteng19
Originally posted by: Pacfanweb
However, on a fuel injected vehicle, this isn't necessarily the case. Most cars nowadays, if you coast for a few seconds, will completely shut off the fuel. So they aren't burning any gas, which is obviously less than the minute amount that it burns at idle.

OK, so if you coast in gear and the computer shuts off the injectors, what keeps the engine turning?
The wheels (via momentum)?
Do the sparks fire?
Your tranny is engages, therefore, the spinning of the wheels, axles, driveshaft, etc. keep the engine turning.
Yes, the plugs fire, but there's no fuel to ignite...just air.
 

Pacfanweb

Lifer
Jan 2, 2000
13,131
54
91
Originally posted by: FrustratedUser
Originally posted by: psteng19
Well, my gas mileage again has plummeted from a consistent 43 mpg to about 35 mpg due to the winter season (mostly gas and warming up).
So being the anal guy I am about fuel efficiency, I'm looking for the best ways to improve fuel economy.

I've read in past threads that coasting in high gear uses less gas than coasting in neutral.
How is that so?
So coasting in 5th gear at 65 mpg with engine about ~2500 RPM is using less gas than coasting in neutral (idle) at 800 RPM?
I know idling uses next to no gas, so coasting at 2500 RPM in gear would have to use like no gas and only the current momentum to keep me going. So the injectors don't pump gas into the chamber when coasting in gear?

What about using cruise control? I see publications telling us to use cruise control to keep RPM's constant (assuming flat roads).
How is it different if we were to keep a constant RPM manually?
Probably not much, if any right?
Actually coasting in gear uses NO gas because the ECU on modern cars turns off fuel delivery.
But not immediately, so TECHNICALLY, you do use a bit of fuel before the PCM cuts the fuel off.
Like I said before, either way you use a minimal amount of fuel. It is really not a concern, just leave your car in gear and coast.
 

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