The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 10-03-2011, 08:20 AM
Colophon Colophon is offline
Guest
 
Join Date: Sep 2002
Why does my car use more fuel coasting in neutral than when in gear?

I recently bought a new (well, second-hand) car. One of the gizmos it has that my old one didn't is a display of current fuel consumption. In the interests of economy (with petrol costing almost 1.40 a litre now) I've been experimenting with how different driving styles affect consumption.

One odd thing I've noticed is that if I shift into neutral and coast down the hill towards my house (I know this is not recommended practice, BTW), the consumption tends to show about 140-150mpg. However, if I leave it in fifth gear and take my foot off the accelerator, cruising down the hill under engine braking, the mileage improves, often going "off the scale" (anything over 199mpg shows as dashes).

Why should this be the case? I always assumed that the engine speed was the important factor, and with the engine braking effect the rpm is far higher (~3000rpm) than the idle speed in neutral.

Last edited by Colophon; 10-03-2011 at 08:24 AM..
Reply With Quote
Advertisements  
  #2  
Old 10-03-2011, 08:24 AM
Ludovic Ludovic is online now
Charter Member
 
Join Date: Jul 2000
Location: America's Wing
Posts: 23,252
The way I've heard it explained in these sort of threads is:

The engine needs to run a certain RPM just in order to stay on, and in order to do this, it need to burn some gas when in neutral.

However, when in gear, the car can get the required RPM from the wheels, so to speak, and that engine revving you hear is not actually using much gas at all -- in fact, less gas than if it were in neutral, because most if not all of the engine RPMs are "powered" by the engine braking.
Reply With Quote
  #3  
Old 10-03-2011, 08:25 AM
jz78817 jz78817 is offline
Guest
 
Join Date: Aug 2010
if your new car has DFCO (deceleration fuel cut-off) the fuel system shuts off while coasting. If you put it in neutral, it can't, so it keeps running at near idle.
Reply With Quote
  #4  
Old 10-03-2011, 08:39 AM
Gary T Gary T is offline
Member
 
Join Date: Mar 2002
Location: KCMO
Posts: 9,435
Don't know for sure, but it might be that with the vehicle in gear the computer recognizes a "deceleration" mode and provides less gas than for the "idle" mode associated with being in neutral.

ETA: Didn't see the other responses before posting. Great minds think alike?

Last edited by Gary T; 10-03-2011 at 08:40 AM..
Reply With Quote
  #5  
Old 10-03-2011, 02:38 PM
AskNott AskNott is offline
Charter Member
 
Join Date: Mar 1999
Location: Anderson, IN,USA
Posts: 13,983
In the era of carburetors, taking your foot off the accelerator pedal would engage "the idle circuit," which gave the engine a rich mixture to keep the engine running. Decelerating in gear used more fuel than idling in neutral. That brought us a folklore that no longer is true.

Today, with computerized fuel injection, little-to-no fuel is used with your foot off the go pedal, whether you are in gear or not.
Reply With Quote
  #6  
Old 10-03-2011, 02:58 PM
Keeve Keeve is offline
Guest
 
Join Date: Aug 2000
Quote:
Originally Posted by Ludovic View Post
However, when in gear, the car can get the required RPM from the wheels, so to speak, and that engine revving you hear is not actually using much gas at all -- in fact, less gas than if it were in neutral, because most if not all of the engine RPMs are "powered" by the engine braking.
If I'm understanding this correctly, then here's a dumbed-down version of it: When you're going downhill in gear, energy can go from the wheels into the engine; but in neutral, the wheels and engine aren't connected, and the energy gets wasted. I don't know enough about cars to know if this really affects the gas mileage, but I'm pretty confident that it does affect charging the battery.
Reply With Quote
  #7  
Old 10-03-2011, 03:00 PM
control-z control-z is online now
Guest
 
Join Date: Mar 2003
Another thing to note, those "instant MPG" readings seem to be on the generous side. Mine in my Mazda indicate I get 34MPG or better, but 27 is what I really get.
Reply With Quote
  #8  
Old 10-03-2011, 03:06 PM
Mangetout Mangetout is online now
Charter Member
 
Join Date: May 2001
Location: England (where it rains)
Posts: 51,003
One thing to note though: when you're coasting in neutral, you may be using a little fuel to keep the engine ticking over, but you're not braking. When you engine-brake in top gear, the fuel system might be off or nearly off, but you're *slowing down more*

These threads invariably overlook the fact that slowing to a stop is not always the objective.

Last edited by Mangetout; 10-03-2011 at 03:07 PM..
Reply With Quote
  #9  
Old 10-03-2011, 03:35 PM
lazybratsche lazybratsche is offline
Guest
 
Join Date: Feb 2006
Quote:
Originally Posted by control-z View Post
Another thing to note, those "instant MPG" readings seem to be on the generous side. Mine in my Mazda indicate I get 34MPG or better, but 27 is what I really get.
Eh, I'd guess that those gauges are accurate, but humans (you and me, both) are lousy at mentally integrating miles per gallon over time. Thus, if you did the math, you could very well be doing 34 miles per gallon when you cruise, but if you properly accounted for short periods of high consumption (say, 10 mpg whenever you accelerate briskly) you would reach the 27 mpg average.
Reply With Quote
  #10  
Old 10-03-2011, 03:43 PM
control-z control-z is online now
Guest
 
Join Date: Mar 2003
Quote:
Originally Posted by lazybratsche View Post
Eh, I'd guess that those gauges are accurate, but humans (you and me, both) are lousy at mentally integrating miles per gallon over time. Thus, if you did the math, you could very well be doing 34 miles per gallon when you cruise, but if you properly accounted for short periods of high consumption (say, 10 mpg whenever you accelerate briskly) you would reach the 27 mpg average.
True, what I actually meant was the display will also show an "Average MPG" that seems to be an overall average, and it's always optimistic.
Reply With Quote
  #11  
Old 10-03-2011, 04:50 PM
wisernow wisernow is online now
Charter Member
 
Join Date: Apr 2004
Location: at my desk
Posts: 683
A related question -

The fuel consumption display has two viewing options, one gives the instantaneous consumption and the other shows the average.

My question is about the second one. What is averaging denominator? It is the fuel consumed divided by what distance? Since the car was first started, or is it more like the average over the last couple of 100 or 1000 kms/miles?
Reply With Quote
  #12  
Old 10-03-2011, 05:23 PM
LawMonkey LawMonkey is offline
Guest
 
Join Date: Mar 2006
Quote:
Originally Posted by wisernow View Post
A related question -

The fuel consumption display has two viewing options, one gives the instantaneous consumption and the other shows the average.

My question is about the second one. What is averaging denominator? It is the fuel consumed divided by what distance? Since the car was first started, or is it more like the average over the last couple of 100 or 1000 kms/miles?
Likely depends on the make/model. My parents' Camrys had this feature, and you could reset the "average" by (if I recall correctly) holding in the little button that you used to cycle through the various metrics. I'd often reset it before doing any significant amount of driving, just out of curiosity--same for the average speed indicator.
Reply With Quote
  #13  
Old 10-03-2011, 07:44 PM
zwede zwede is offline
Guest
 
Join Date: Mar 2001
Quote:
Originally Posted by control-z View Post
True, what I actually meant was the display will also show an "Average MPG" that seems to be an overall average, and it's always optimistic.
Some cars are optimistic, some pessimistic. My experience:

2005 BMW = 8% optimistic (gauge read more than actual)
2006 Chevy = 1-2% pessimistic
2012 Ford = 4% pessimistic
Reply With Quote
  #14  
Old 10-03-2011, 08:20 PM
GameHat GameHat is online now
Guest
 
Join Date: Dec 2007
Quote:
Originally Posted by Colophon View Post
Why should this be the case? I always assumed that the engine speed was the important factor, and with the engine braking effect the rpm is far higher (~3000rpm) than the idle speed in neutral.
Yeah but in fifth gear there should be negligible engine braking. If I'm at a decent speed (<30 mph) the tach is usually lower in fifth gear (I'm guessing from memory maybe 1800 rpm) than in neutral (guessing again, 2300 rpm)

This is in a manual transmission (Mustang GT.) YMMV, literally.
Reply With Quote
  #15  
Old 10-03-2011, 08:41 PM
Rick Rick is offline
Charter Member
 
Join Date: Aug 1999
Posts: 15,490
With modern fuel injection systems any time you take your foot off the gas the injectors will turn off until a preset RPM is reached and then they will turn back on. On the cars I work on the turn back on point is about 1800 RPM cold and say 1300 RPM hot.
So if you take your foot off the gas in neutral the engine will get zero fuel for that split second it takes to get near idle. If you are in gear and coasting as long as the RPM is above the resume point no fuel is being injected.

ETA: engine braking as nothing to do with it, it is a function of engine RPM and will happen in neutral as well as gear.
Flat ground as well as hills.
__________________
Remember this motto to live by: Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather one should aim to skid in sideways, chocolate in one hand, glass of Scotch in the other, your body thoroughly used up, totally worn out and screaming "WOO HOO! Man, what a ride!"

Last edited by Rick; 10-03-2011 at 08:43 PM..
Reply With Quote
  #16  
Old 10-03-2011, 09:08 PM
Arrendajo Arrendajo is offline
Guest
 
Join Date: May 2010
The mileage computer in my Fit is always higher than my calculated mileage by about 5 mpg.
Reply With Quote
  #17  
Old 10-04-2011, 09:53 AM
Colophon Colophon is offline
Guest
 
Join Date: Sep 2002
Quote:
Originally Posted by GameHat View Post
Yeah but in fifth gear there should be negligible engine braking. If I'm at a decent speed (<30 mph) the tach is usually lower in fifth gear (I'm guessing from memory maybe 1800 rpm) than in neutral (guessing again, 2300 rpm)

This is in a manual transmission (Mustang GT.) YMMV, literally.
Your car idles at 2300rpm? I've never driven a Mustang but that seems extremely high. Most cars I've driven idle in the 800rpm sort of range. If you're coasting in neutral, surely the engine should just be idling?

Going downhill at around 50mph in 5th gear the revs are about 3000rpm, from memory. In neutral they're below 1000. (VW Passat, manual transmission).

wisernow: at least on my car, the average consumption is calculated since the last reset. I tend to reset it each time I fill up, if I remember.
Reply With Quote
  #18  
Old 10-04-2011, 09:17 PM
GameHat GameHat is online now
Guest
 
Join Date: Dec 2007
Quote:
Originally Posted by Colophon View Post
Your car idles at 2300rpm? I've never driven a Mustang but that seems extremely high. Most cars I've driven idle in the 800rpm sort of range. If you're coasting in neutral, surely the engine should just be idling?

Going downhill at around 50mph in 5th gear the revs are about 3000rpm, from memory. In neutral they're below 1000. (VW Passat, manual transmission).

wisernow: at least on my car, the average consumption is calculated since the last reset. I tend to reset it each time I fill up, if I remember.
Gah, now I've got to check it. I'll try it tomorrow. Like I said, I was going from memory.

I'm pretty sure though that at 800 rpm my Mustang is sputtering and close to stalling. But I gotta check that, too.
Reply With Quote
  #19  
Old 10-04-2011, 09:45 PM
AaronX AaronX is offline
Guest
 
Join Date: Feb 2011
Consumption might be higher in neutral, but what about total consumed? If you coast faster in neutral, the total might still be lower.
Reply With Quote
  #20  
Old 10-04-2011, 10:29 PM
jz78817 jz78817 is offline
Guest
 
Join Date: Aug 2010
Quote:
Originally Posted by GameHat View Post
Gah, now I've got to check it. I'll try it tomorrow. Like I said, I was going from memory.

I'm pretty sure though that at 800 rpm my Mustang is sputtering and close to stalling. But I gotta check that, too.
yeah, if you're like in fifth gear and trying to lug it at 800 rpm, maybe. no street car idles all the way up at 2300 rpm. both of my Mustangs idled at ~700 rpm. bigger engines idle as low as 500-600.
Reply With Quote
  #21  
Old 10-05-2011, 06:59 AM
Rick Rick is offline
Charter Member
 
Join Date: Aug 1999
Posts: 15,490
Quote:
Originally Posted by GameHat View Post
Gah, now I've got to check it. I'll try it tomorrow. Like I said, I was going from memory.

I'm pretty sure though that at 800 rpm my Mustang is sputtering and close to stalling. But I gotta check that, too.
Your memory is bad. No car idles at 2300.
__________________
Remember this motto to live by: Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather one should aim to skid in sideways, chocolate in one hand, glass of Scotch in the other, your body thoroughly used up, totally worn out and screaming "WOO HOO! Man, what a ride!"
Reply With Quote
  #22  
Old 10-07-2011, 10:23 PM
GameHat GameHat is online now
Guest
 
Join Date: Dec 2007
Quote:
Originally Posted by Rick View Post
Your memory is bad. No car idles at 2300.
I looked today. Yep, you guys are right. I'm not a car guy. And I have a bad memory. My car idles somewhere between 600-800 RPM.

/Really can drive a stick like a maniac
//Very little mechanical intuition or know-how
///I'm an engineer, but chemical. Mechanical just confounds me. Electrons are easy. Gears are baffling.

Last edited by GameHat; 10-07-2011 at 10:26 PM..
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 04:08 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.