How To Increase Your Gas Mileage

ChrisSlicks · 04-05-2009, 09:45 PM

Quote:

Originally Posted by frost

^ interesting you say that, because when I'm looking at my instant fuel gauge, and I drop it into neutral, I will go into the 40mpg+ area, but it stays in the 20s if I just let off.
Thoughts?

It goes 40+ in neutral but should go off the chart (60+) when you lift off completely.

frost · 04-05-2009, 09:47 PM

Quote:

Originally Posted by ChrisSlicks

It goes 40+ in neutral but should go off the chart (60+) when you lift off completely.

I'll have to give this a better look tomorrow, sure would be handy to stay in gear.

dad · 04-05-2009, 10:15 PM

Coasting in neutral consumes more fuel. In neutral, the injectors fire to keep the engine at idle, so you are burning fuel. Coasting in gear, the ECU switches to deceleration ... which happens to have an injector pulse width of zero.

monthtrial · 04-06-2009, 11:46 PM

Quote:

Originally Posted by dad

Coasting in neutral consumes more fuel. In neutral, the injectors fire to keep the engine at idle, so you are burning fuel. Coasting in gear, the ECU switches to deceleration ... which happens to have an injector pulse width of zero.

FYI: I'm no mechanic nor an auto engineer... just a dude using common sense and the fuel gauge read-out on my car...

I still don't understand why coasting in neutral would burn more fuel?

Are you suggesting that simply lifting OFF the gas consumes less fuel than placing the car in neutral to idle @ 750 RPM? I think there's a direct correlation between fuel consumption and RPM range - unless you're bogging the engine or loading it.

If your theory is correct, I could be driving down hill in second gear @ 6,000 RPM with my foot off the gas pedal (yet maintaining speed due to gravity & slope/grade) and save more fuel than popping it in neutral.

Very, very arguable, irrespective of however technical you may wanna get w/ injectors or widths.

Sure, there may be a point at which the speed one drives w/ the foot off the fuel pedal conserves fuel, but gearing and velocity have to be factored in and there comes a point where idling and maintaining a certain momentum (vs. losing momentum due to engine speed slowing the car) is more efficient.

Can you refute my non-technical assessment? If so, please do so in plain english.

wstar · 04-07-2009, 01:14 AM

Well, if he's right about the fuel getting cut during engine braking, then yes engine braking would save more gas than coasting. However, I find this hard to believe. Cut fuel = no more combustion. The drivetrain would simply be spinning the engine and sucking air through the pistons for compression braking, but without any ignitions happening (which means really they could cut the spark plug current off too). I suppose in theory this sounds possible, but I would think I would hear the difference between an engine that's undergoing combustion and one that isn't, and I would think the transition between the two states would be rather abrupt and noticeable. Sure seems pretty smooth to me.

I'm not saying it isn't true, I'm just saying I find it hard to believe

ChrisSlicks · 04-07-2009, 11:28 AM

The mechanical noise of all the engine parts moving far exceeds any noise the actual ignition makes.

In full lift off the injectors do not fire (there may be no spark either, but I'm not sure). It matters not the gear or the RPM's as long as the engine is above idle speed. As you approach idle the injectors will begin firing again to prevent stall.

You do get great mileage in neutral as well because idle fuel burn is minimal and there are times where that is going to save more fuel than lift off if you can do it for longer. But if you're going down hill you absolutely should be in gear.

monthtrial · 04-08-2009, 12:41 AM

Theory and engine mechanics are one thing... real-life dynamics and physics are another. With an engine stationed @ a laboratory, I can see that an engine will conserve fuel by being in gear, however, not on our highways...

What I'm getting at is that the gear you're in makes a difference! To elaborate, lets assume we're coasting down a hill. Without using the gas pedal whatsoever, first gear will be less efficient to coast in than 2nd gear, than 3rd, gear, etc.... this is due to the ability to gain greater speeds. Even coasting in 6th gear will eventually reach the point where the car will NOT gain more speed. Popping the car in neutral in this case SHOULD result in increased speed (since the engine RPM's don't hold the car at a steady speed) and this should result in more distance covered in less time (speed). This is inherently what fuel mileage equations take into account...

So as I see it, when variables of physics are thrown into the equation, the whole concept of fuel savings needs to be reevaluated.

dad · 04-08-2009, 12:53 PM

Quote:

Originally Posted by wstar

Well, if he's right about the fuel getting cut during engine braking, then yes engine braking would save more gas than coasting. However, I find this hard to believe. Cut fuel = no more combustion. The drivetrain would simply be spinning the engine and sucking air through the pistons for compression braking, but without any ignitions happening (which means really they could cut the spark plug current off too). I suppose in theory this sounds possible, but I would think I would hear the difference between an engine that's undergoing combustion and one that isn't, and I would think the transition between the two states would be rather abrupt and noticeable. Sure seems pretty smooth to me.

I'm not saying it isn't true, I'm just saying I find it hard to believe

There is a distinct change in pitch between accelerating and coasting.

wstar · 04-08-2009, 01:48 PM

Quote:

Originally Posted by dad

There is a distinct change in pitch between accelerating and coasting.

Agreed. I just find it hard to believe that what my ears are hearing during engine braking is just restricted airflow pumping through the cylinders with no combustion.

04-05-2009, 10:15 PM	#3 (permalink)
dad Grand Prix of Endurance Join Date: Nov 2008 Location: Tennessee Posts: 3,476 Drives: Mulsanne Straight Rep Power: 27	Coasting in neutral consumes more fuel. In neutral, the injectors fire to keep the engine at idle, so you are burning fuel. Coasting in gear, the ECU switches to deceleration ... which happens to have an injector pulse width of zero. __________________ We can do without any article of luxury we have never had; but once obtained, it is not in human nature to surrender it voluntary.

04-07-2009, 01:14 AM	#5 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3596	Well, if he's right about the fuel getting cut during engine braking, then yes engine braking would save more gas than coasting. However, I find this hard to believe. Cut fuel = no more combustion. The drivetrain would simply be spinning the engine and sucking air through the pistons for compression braking, but without any ignitions happening (which means really they could cut the spark plug current off too). I suppose in theory this sounds possible, but I would think I would hear the difference between an engine that's undergoing combustion and one that isn't, and I would think the transition between the two states would be rather abrupt and noticeable. Sure seems pretty smooth to me. I'm not saying it isn't true, I'm just saying I find it hard to believe __________________ 7AT Track Car! Journal thread / Car setup details

04-08-2009, 12:41 AM	#7 (permalink)
monthtrial Base Member Join Date: Jan 2009 Location: Fullerton Age: 49 Posts: 46 Drives: '13 370z Rep Power: 18	Theory and engine mechanics are one thing... real-life dynamics and physics are another. With an engine stationed @ a laboratory, I can see that an engine will conserve fuel by being in gear, however, not on our highways... What I'm getting at is that the gear you're in makes a difference! To elaborate, lets assume we're coasting down a hill. Without using the gas pedal whatsoever, first gear will be less efficient to coast in than 2nd gear, than 3rd, gear, etc.... this is due to the ability to gain greater speeds. Even coasting in 6th gear will eventually reach the point where the car will NOT gain more speed. Popping the car in neutral in this case SHOULD result in increased speed (since the engine RPM's don't hold the car at a steady speed) and this should result in more distance covered in less time (speed). This is inherently what fuel mileage equations take into account... So as I see it, when variables of physics are thrown into the equation, the whole concept of fuel savings needs to be reevaluated. Last edited by monthtrial; 04-08-2009 at 01:04 AM.

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04-07-2009, 11:28 AM	#6 (permalink)
ChrisSlicks A True Z Fanatic Join Date: Mar 2009 Location: North East Posts: 6,203 Drives: 09 370Z Sport M6 Rep Power: 655	The mechanical noise of all the engine parts moving far exceeds any noise the actual ignition makes. In full lift off the injectors do not fire (there may be no spark either, but I'm not sure). It matters not the gear or the RPM's as long as the engine is above idle speed. As you approach idle the injectors will begin firing again to prevent stall. You do get great mileage in neutral as well because idle fuel burn is minimal and there are times where that is going to save more fuel than lift off if you can do it for longer. But if you're going down hill you absolutely should be in gear.