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Discussion Starter #1
If you had a challenger sitting in traffic and it’s an automatic, do you consume more gas while in auto or would it help to put in manual or neutral to conserve?


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If you had a challenger sitting in traffic and it’s an automatic, do you consume more gas while in auto or would it help to put in manual or neutral to conserve?


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Neutral. Since your car's idle is controlled by a computer, it takes slightly more fuel to maintain idle speed if the engine is exerting any force on the rear wheels. That force exerted isn't free.

You also conserve your brakes by putting the car in neutral below 4 mph, the speed at which the car will travel as it idles in first. It's easier for the brakes to stop the car at any speed below 4 mph if they're not fighting the engine.
 

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That’s what I thought but wasn’t sure if computers in cars these days were smart enough to not throttle while stopped.


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I'd think the possible need to accelerate suddenly, and the possible effect on the transmission always shifting in and out of neutral, would negate any possible advantage.

A Guy
 

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I find that she's more conservative on fuel while in automatic, not 100% certain, but it seems that way. In manual the engine's 'Economy' function doesn't seem to activate and all 8 cylinders are on line. While sitting in traffic at a stop light there really doesn't seem to be much of a difference. Never considered shifting into neutral to take any load off the engine. The Hidden Evic function does show real time percentages of fuel being drawn from either tank from what I understand. Why not run a comparison to see if there is any difference between automatic, neutral and manual?
 

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I find that she's more conservative on fuel while in automatic, not 100% certain, but it seems that way. In manual the engine's 'Economy' function doesn't seem to activate and all 8 cylinders are on line. While sitting in traffic at a stop light there really doesn't seem to be much of a difference. Never considered shifting into neutral to take any load off the engine. The Hidden Evic function does show real time percentages of fuel being drawn from either tank from what I understand. Why not run a comparison to see if there is any difference between automatic, neutral and manual?


Great post, I’ll recommend to the guy that is having issues. I myself have a M6 so I’m not in the same boat...thanks for the great insight.


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I'm guessing that those of us who drive cars with the manual aren't ever concerned with gas consumption while stopped at a light. Many of them are in first gear and constantly, slightly revving the engine. When stopped on an upward grade, they're likely to be revving a little and slipping the clutch to "hold" the car in it's spot. It's just more fun to do that than letting the electronics do the work.
 

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I find that she's more conservative on fuel while in automatic, not 100% certain, but it seems that way. In manual the engine's 'Economy' function doesn't seem to activate and all 8 cylinders are on line. While sitting in traffic at a stop light there really doesn't seem to be much of a difference. Never considered shifting into neutral to take any load off the engine. The Hidden Evic function does show real time percentages of fuel being drawn from either tank from what I understand. Why not run a comparison to see if there is any difference between automatic, neutral and manual?
You said you're "not certain" about mileage while the trannie is in auto. I've tested this and can affirm that the car is indeed more fuel efficient in auto. You mentioned one reason, the MDS. However, this doesn't activate in lower gears during stop and go traffic, as you know. MDS is not a factor at the stop light. But there's another factor. The automatic keeps the engine operating at minimum RPMs at light throttle. It's got me in 3rd gear before I reach 15mph at light throttle. This, combined with running on four cylinders at higher speeds, makes a huge difference in mileage. The OP question about gas usage while stopped is only going to amount to a small fraction of gas savings and as A Guy wisely pointed out, is not advisable for most drivers. We daydream too much while stopped.
 

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You said you're "not certain" about mileage while the trannie is in auto. I've tested this and can affirm that the car is indeed more fuel efficient in auto. You mentioned one reason, the MDS. However, this doesn't activate in lower gears during stop and go traffic, as you know. MDS is not a factor at the stop light. But there's another factor. The automatic keeps the engine operating at minimum RPMs at light throttle. It's got me in 3rd gear before I reach 15mph at light throttle. This, combined with running on four cylinders at higher speeds, makes a huge difference in mileage. The OP question about gas usage while stopped is only going to amount to a small fraction of gas savings and as A Guy wisely pointed out, is not advisable for most drivers. We daydream too much while stopped.


This guy I’m asking the question for is getting 8-9mpg in city traffic while commuting 8.6m in 20-25m. So basically it sounds like he’s doing a lot of sitting and I was wondering if he could switch the A8 to manual or neutral to see if gas consumption improved. In my M6, on the worst possible conditions, I don’t think I’ve ever gone below 11mpg. Hence the question, would that slight tug while stopped in an A8 while stopped cause a 2mpg drop in fuel consumption.


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This guy I’m asking the question for is getting 8-9mpg in city traffic while commuting 8.6m in 20-25m. So basically it sounds like he’s doing a lot of sitting and I was wondering if he could switch the A8 to manual or neutral to see if gas consumption improved. In my M6, on the worst possible conditions, I don’t think I’ve ever gone below 11mpg. Hence the question, would that slight tug while stopped in an A8 while stopped cause a 2mpg drop in fuel consumption.
Ughh. Just typed up a lengthy response here, but the computer ate it because the website logged me out while I was typing it.
In a nutshell, No; putting the automatic in manual mode won't save anything. It will be more efficient in automatic. Assuming the two of you have the same car, I highly doubt that the difference is due to the automatic. If anything , he should get better mileage than you do with the manual, especially if his MDS kicks in often. The neutral idea will likely save him some, but not as much as he's hoping. I'm thinking maybe 1/10 of a 1 mpg. I'm just guessing here.

Driving style or mistakes in mileage calculations are most likely the culprit. For example, a huge factor in gas mileage is how often the car is started when it's cold. Two trips to the mailbox each day, at 1/4 mile per trip, will kill overall gas mileage. An engine gets like 1 mpg during the first 30 seconds it's running, especially in colder weather. Multiple short trips like this during the week can make a huge difference.
 

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Here's an article that will give you food for thought on the subject of sitting a car off and restarting it and how much gas that saves. It's not exactly the same thing we're talking about here but it offers insight into just how much gas is actually being used during stop/start traffic.

Personally, I can't imagine any measurable difference in fuel consumption between have the transmission remain in gear or changing it to neutral at a stop light. I've read articles that say the wear and tear on the mechanics would far outweigh any benefit in gas savings. I believe that to be true.

https://www.edmunds.com/car-reviews/features/do-stop-start-systems-really-save-fuel.html
 

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It boils down to:

Yes you save some gas.
Yes the gas you save so minuscule as to be immeasurable.

Something else to think about (cause I do this neutral thing, its a habit), when you are in neutral and stop, then let off the brake to go, you car starts to roll (but not very fast) you cannot put it in drive without stepping on the brake, often causing the car to stop. Much to the annoyance of the driver behind you. I am currently trying to come up with a system that would use a microswitch and relays to flash the brakelight (and fool the computer into thinking I am stepping on the brake) when I push the button on the shifter.
 

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It boils down to:

Yes you save some gas.
Yes the gas you save so minuscule as to be immeasurable.

Something else to think about (cause I do this neutral thing, its a habit), when you are in neutral and stop, then let off the brake to go, you car starts to roll (but not very fast) you cannot put it in drive without stepping on the brake, often causing the car to stop. Much to the annoyance of the driver behind you. I am currently trying to come up with a system that would use a microswitch and relays to flash the brakelight (and fool the computer into thinking I am stepping on the brake) when I push the button on the shifter.
You're gonna screw with the car's transmission to satisfy a habit you know is pointless?
 

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I'm guessing that those of us who drive cars with the manual aren't ever concerned with gas consumption while stopped at a light. Many of them are in first gear and constantly, slightly revving the engine. When stopped on an upward grade, they're likely to be revving a little and slipping the clutch to "hold" the car in it's spot. It's just more fun to do that than letting the electronics do the work.
It's one of my pet peeves. For me, my only concern about gas consumption is to be sitting at a traffic signal waiting for it to change when I've got the only vehicle at the intersection. It should be legal to go through an intersection or make a left turn, after coming to a full stop, if no other vehicles are present, just the same as if one was at an intersection with a stop sign.
 

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I'd think the possible need to accelerate suddenly, and the possible effect on the transmission always shifting in and out of neutral, would negate any possible advantage.

A Guy
Totally agree with this. Might as well shut the car off at lights if you’re that worried about it.

Some modern cars do shut off instead of idling....
 

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Neutral. Since your car's idle is controlled by a computer, it takes slightly more fuel to maintain idle speed if the engine is exerting any force on the rear wheels. That force exerted isn't free.

You also conserve your brakes by putting the car in neutral below 4 mph, the speed at which the car will travel as it idles in first. It's easier for the brakes to stop the car at any speed below 4 mph if they're not fighting the engine.
Then there's the wear on AT from the clutches engaging / disengaging from shifting D / N / D needlessly.
 

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I'm guessing that those of us who drive cars with the manual aren't ever concerned with gas consumption while stopped at a light. Many of them are in first gear and constantly, slightly revving the engine. When stopped on an upward grade, they're likely to be revving a little and slipping the clutch to "hold" the car in it's spot. It's just more fun to do that than letting the electronics do the work.
Until the light goes green or I'm at the intersection and next to go, I'm in Neutral, holding the brakes...I don't slip the clutch to hold on a hill. Otherwise, that clutch is getting wear and heating up...

I lived in San Francisco and drove 'stick cars all the time - pretty much have hills and slopes on most streets there.
 
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