if you do a lot of stop and go driving, that is where you're using most of your gas. while it's true that idling you get 0 mpg, you actually use more during acceleration, you're just adding distance along with it.

 

In a typical urban envirnoment you use about 10% of gas while stationary at idle, much more if its an auto and you leave it in D (just shifting into N at every stop will save you about 5%), and also more if you have AC on (Percentage wise its worse at idle than when the engine is under load in more economical areas of the speed/load range)

Simon

 

... you use about 10% of gas while stationary at idle, much more if its an auto and you leave it in D (just shifting into N at every stop will save you about 5%)...

Why is that?

 

i just found the answer to my question -- apparently a car uses half a mile worth of gas for every minute it sits idle. so, if i sit idle for 20 minutes in traffic, my car uses half a gallon of gas (provided it gets 20 miles/gallon in heavy traffic).

this means that i waste $10 week in gas while sitting idle in traffic (since i sit idle for 100 minutes a week). that is 25% of my gas budget WASTED! just amazing.

 

Kimold. I'm doing some research on this topic. Where did you get your information? I think you somehow mis-read the formula. By your math your car burns 1.5GpH at idle. That means your Subaru burns about 1.5x the gas than an International or Kenworth big rig truck does diesel. That can't be right. (if it is then Subaru's gas milage is really, really, really horrible.)

However, I want to know where you got the "half a mile worth of gas for every minute it sits idle." That would be dependant on the mileage you expect to be travelling at. Highway or City for example. But then since the mileage is different that means consumption at idle would be different depending on the expected use of the vehicle.

Anyway, something isn't adding up. What's your source?

 

Oh dang. I never realized it was that much.

 

Mark my words, the next addition to OEM gas-savings technology will be digitally controlled engine shutdown and/or shift-into-neutral/park circuit gizmos.....it makes sense, you know.

Prius's have had this on their drivetrains since the hybrid's first gen and it seems to work well enough............

I have been shifting into neutral for the past several years, more or less.....much more consistently now that oil has spiked again. My last highway trip got me 29.96 MPG which is darned good for a safe, AWD car.........steady 60 MPH and easy on the gas pressure did it.

Steve

 

Damn Steve, you called it. Who knows if you'll ever see this. And I made an account just to say this.

Nowadays there's auto engine start and stop (no idea what its technically called) all over the place, not to even mention Tesla.

 

^^kinda like what honda does with valve shutdown at idle?

 

Nearly all (if not all now) BMW/Mini's in the EU market have the 'idle stop' feature already, its just that as gas is so cheap (relatively) in the US there has been less interest from over there...and before you bleat about $4/gallon, remember we in the UK are paying about $7.8 equivalent.....(£112.9/litre, 4.54litre to ImpG, £/$ 1.9 and USg=0.8ImpG)

Simon

 

Keeping the AT in "D", when stopped, puts the drive train in a slight load situation. Putting the selector in "P" or "N" releases the engine and allows it to idle without the load caused by the need to hold your foot on the brake......

Steve

 

Keeping the AT in "D", when stopped, puts the drive train in a slight load situation. Putting the selector in "P" or "N" releases the engine and allows it to idle without the load...

So by idling faster without the load, the engine uses more gas when stopped in N.

 

NO...it uses LESS......as has been said twice!

Simon

 

With my 2008 4EAT, when the engine is hot and at a stop light, the RPMs in Neutral and Drive are the same.

In older cars where the Neutral RPMs are high and dragged down when shifted into Drive, you would think that the higher RPMs in Neutral be wasting more gas and the lower RPMs in Drive would be saving gas.

But the constant throttle position at idle, with no foot on the pedal, is putting the same amount of air and fuel into the engine regardless of gear. When the RPMs drop when shifted into Drive, the missing RPMs are converted into some heat in torque converter. The same amount of fuel that was used to make the higher RPMs in Neutral, now makes heat in the transmission in Drive.

So whether you use Neutral or Drive at stops, the amount of fuel used is the same.

 

With no use of the throttle, foot off the gas, how can the engine use less gas when it is idling faster?
To save gas, leave the transmission in D when stopped. Shifting to N only increases the RPMs and wastes gas.

i understand what you're saying but i can't really describe why that doesn't work. here is what the rookie posted on that "Percentage wise its worse at idle than when the engine is under load in more economical areas of the speed/load range". maybe he will be able to elaborate more to help you understand this better.

 

Automatic Neutral Control--A New Fuel Saving Technology for Automatic Transmission

Some ATs incorporate this sort of technology to automatically go into a "virtual neutral" when sitting still in D with the brakes applied. I don't know for sure whther the Subaru AT has this feature or not, the sales brochure for mine claims that it does, but I can't detect it working.

You will notice that that the wording of the patent supports the Rookie's assertion that (in the absence of such a device) you would save fuel by shifting to N. Which is common sense unless you slept through physics lessons at school - the energy to heat up the fluid is coming from somewhere.

-- Steve

 

For what it is worth this is what I ferreted out.

Expect your average modern passenger vehicle to burn around 0.3GPH while sitting at idle.


SOURCE BASIS GPH
Newsgroup 98 4CYL (computer) 0.2910
Anti-Idle website 30 Min <= 1/10Gal 0.2000
Anti-Idle website 10 MIN = 0.026Gal 0.1560
epa.gov study on trucks and locomotives 0.8000
Newsgroup Big rigs 1.0000
Newsgroup personal experience 0.3000
Newsgroup Large carbeurated V8 0.5000
Newsgroup computer reading 0.3000
Newsgroup Big rigs 1.0000

 

If there is no load on the engine when stopped in D, how come the damn thing moves when you come off the brake pedal, thats the extra load now moving the car...if there was only enough airflow to maintain enough torque to maintain idle speed (that is overcome internal losses and paracitic losses), there would be no extra torque to move the car!

You sure it idles at 400rpm...whatever the valve opens to admit more air to compenaste for the load, ISCV is a common acronym, but IACV is more accurate as letting more or less air in doesn't necesarily change the speed, if you don't have an IACV the damn thing would stall when you shifted into D, thats one of the things I've been responsible for on various products I've worked on over the years - getting the airflow right to get the right N-D shift feel!

Some old carb cars had no means of jacking the airflow so relied on a high idle speed in N to compensate!

A little knowledge is a dangerous thing - be careful out there!
Simon

 

If there is no load on the engine when stopped in D, how come the damn thing moves when you come off the brake pedal, thats the extra load now moving the car...if there was only enough airflow to maintain enough torque to maintain idle speed... there would be no extra torque to move the car!
You sure it idles at 400rpm......

When hot, my Forester idles with the tach needle entirely below and not touching the thin white line that represents 500 RPM.
Shifting back and forth between D and N produces no change in engine RPM or sound, no perceptible difference in load.
When idling in D, if on a smooth and level surface, the car will begin to inch forward unless the brake pedal is lightly pressed.
So that is called "moving the car" and takes 5% more fuel without using more throttle or RPM? Surely one must be given the leeway to doubt the 5%.

 

I don't have a Forester auto, but UK cars idle just below the 750rpm line, 700rpm, 400 rpm sounds very low (much lower than any other 4 cylinder I've worked on!)

It moves, therefore it has more load...at last! How you can say only 5% is beyond me, it takes a fair bit of effort to push a car after all!

Simon

 

I know for a fact that he new Mazda 3 will shift to Neutral from Drive when stopped as a fuel saving measure. I was speaking to a Mazda Master Mechinic about the car a couple weeks ago regaurding its 40 mpg rating and how they acvhieved it.

I can certainly tell a difference in my FXT when stopped if I move it from Drive to Neutral. If you have an ultra guage look at the Manifold Pressure. When stopped at idle in D it should read in the mid 6's and when in N it should read in the mid to low 5's.

I am not in a ton of stop and go traffic so I don't know how much difference it makes but I still do it anyway.

I remember first noticing how much gas is burned at idle in my Volvo 850 Turbo. The mpg gauge would drop about a 1/10th of a mpg every 5 minutes when sitting idle with the car in D.

 

... I remember first noticing how much gas is burned at idle in my Volvo 850 Turbo. The mpg gauge would drop about a 1/10th of a mpg every 5 minutes when sitting idle with the car in D.

How did this compare to sitting at idle with the car in P(ark)?

However, I don't understand how the Volvo's computer gauge could display a mpg when the car is not moving.

There can be no instant running mpg calculation by the car's computer when the car is not moving, as there is no "miles" input.

However, a ScanGuage can show the fuel consumption in GPH at any time, such as sitting at idle, either in D or P.

 

What about wear and tear on the transmission shifting into neutral every time you stop...maybe 50 times a day to shift back and forth or more ?

This certainly would negate any fuel savings over the long term I believe.

 

I'm guessing the Ovlov had a short term averaging, without knowing what the time base was, the info is kind of meaningless.

As a guide, idling in drive will increase fuel used at idle by between 80 and 125%.

Not sure what wear occurs in the trans from shifting into neutral and back into drive.....nothing you even see over 150,000 miles if you tested I'm sure!

Simon

 

... As a guide, idling in drive will increase fuel used at idle by between 80 and 125%....

According to the ScanGuage, my auto Forester uses the same amount of fuel idling in Drive as idling in Park.

In Drive: idle display fluctuates between 604 and 616 RPM, usage display fluctuates between .24 and .28 GPH.
In Park: idle display fluctuates between 692 and 712 RPM, usage display fluctuates between .25 and .28 GPH.

In either Drive or Park, the idling engine uses about 1/4 of a gallon per hour.

 

n=1

 

I know I see my boost gauge pick up a bit less vacuum when switched back into D from N.

 

RPMS are the same in N and D because the idle control valve
In N, the engine spins freely, with no load thus burning less gas

 

Wow this thread goes back many years, with a 10-year gap between the last 2 posts. Anyway, there is not much difference in fuel usage idling in N compared to idling in D. Also, there is no additional load on the engine caused by pressing the brake pedal down unless there is a vacuum leak or some kind of electrical problem. Once the brake pistons stop moving (the pressure is built), there is no more load. I was in another discussion elsewhere and we talked about having to press the brake pedal to start the engine. I said there was additional load, making it harder to crank the engine, and I was corrected. (We were talking about vacuum brakes, not electric.) I used to have a car that the idle speed decreased on when I pressed the brake pedal harder, and that's why I thought there was additional load. But that car was a beater and likely had a vacuum leak or other problem. Source: