Idling in N vs D observations

[stimpsonjcat]

Before I got my scangauge I had done a little testing with my generic ODB reader and had noticed that the % load indicator did show a significant difference between idling in gear vs D in my automatic. So while I don't shut off my engine at lights, I do shift into N if I know it will be a long light.

I have retested this with the scangauge by timing the amount of time it takes for my overall tank average to drop a tenth of a MPG while sitting still in N vs D.

The difference is about 30%. Which is to say that sitting at a light in N costs you 70% of sitting there in D.

[Phantom]

I would look at gal per hr difference I'm not sure exactly what your results were due to the wording, but on my car shifting in to N the gph increases for a short time then slowly drops down to use slightly less than when in D. I do not bother to shift into N when at a light as I will need to be sitting for about a minute for it to matter.

[brucey]

I've found the only difference whether the transmission is in Neutral or Drive with the engine idling is if I'm on the brakes or not. Holding them down brings up the gph by .02 usually. So I just use the emergency brake if it's going to be a short stop. No paying for the brake light use out of my gas tank!

[Kodak]

Any rpm difference stimpson?

[stimpsonjcat]

Hmmm...i will revisit this and check the gph also.

My basic premise was that if the load drops then the fuel use should also.

I had noticed that on short trips the avg usage tends to drop fast when stopped, so i just timed how fast it dropped in N vs D.

It is probably the same math either way, but i will go look at the gph display and we shall see.

Fun tool to play with!

[capnbass91]

I think it really depends on the car, I've seen people say their gph goes down in N and others say it's lower in D.
For example, in my car in D gph is about .45-.49, while in N it usually stays above .50.

[bwilson4web]

It is likely to be a function of the vehicle warm-up state, somewhat specific to the make and model. In our 2003 Prius:

• coolant below 70C - engine will run unless in "R"
• unloaded "N" idle - runs about 0.30 gallons/hr
• loaded "D" idle - runs about 0.60 gallons/hr
• sometimes when stopped - "R" and pulse engine turns engine off
• coolant 70C and above - engine can "auto off" following the multi-stage warm-up

Regardless of which car, the ScanGauge or an OBD scanner that shows mass air flow is needed to track the fuel burn. Then experiment and find out what works.

Bob Wilson

[stimpsonjcat]

New testing, fully warmed vehicle.

-Idling in N shows gph = 0.18
-Idling in D shows gph = 0.22

Quick math tells me I wait at between 6 and 12 minutes worth of lights per day.

So best case savings = 0.2 hours of idle * 0.04 gph delta = 0.008 g/day saved.

At 240 days a year driving this is 1.92 gallons.

Hmmm...doesn't seem at all worth the effort.

I will abandon this method for a few days and check overall mpg for effect.

[Nevyn]

You IDLE at LIGHTS???

[Old Tele man]

...the above discussion is one reason GM has the 6T40 automatic transmission used in the Cruze automatically shift into "Neutral" when stopped at stop lights, to reduce engine "loading" and thus improve MPG (slightly).