Auto in N question - Fuel Economy, Hypermiling, EcoModding News and Forum

Doofus McFancypants · 08-05-2008, 11:37 AM

The family and I went to see my Brother in NC this past weekend
We took the PT Crusier - auto tranny.
I did hook up the SG so i could see how we did.
I gathered data on the car at several speeds to get a feel for the impact MPH on MPG

While doing this - i also wanted to see if coasting in gear was better or worse then costing in Neutral.

From a consumptiuon point of view - coasting in N seemed to use slightly LESS fuel than coasting in gear. I suspect the PT does not have a fuel shut off as the MPG did not skyrocket.
Both cases had the instant MPG jump to about 75 ( N hit 80 a few times) hence i called it SLIGHTLY less fuel.

I wanna play with it some more and get more data - a little more contolled than "30 minutes of data crusing across SC)

Here is my observation and question.

When i shift the tranny into N - I expected the RPM to drop to Idle - which is what the Manual in my Altima does... but it did not?

going 65 mph was about 2400 rpm or so. shift to N - engine stayed near 2400 RPM ! HUH? Both the TACH and the SG said this..

the MPG did go up so i know less fuel is going IN -
Is this normal for an AUTO? Am i thinking "Manual Tranny" here?

I do not drive the Crusier much ( wifes car) so i had not played with it before. but am a little confused at this.

Steve

azraelswrd · 08-05-2008, 11:49 AM

Wow, that is interesting. I can only speak from my own experiences with the Scion XA (automatic) and when I go to N from 45 (since that's where I usually drive at) my RPMs take a dive from 1800-1900 down to 700-900. MPG gains are immediate (instant and current trip) but I'm trying to get into the habit of DFCO coast first then N-coast once I'm below the minimum 1200-1400 rpm to make the most out of fuel consumption.

BrianAbington · 08-06-2008, 10:49 AM

it may be an idle controler problem

bryn · 08-06-2008, 11:30 AM

mine drops to idle also.
what happens if you rev the engine a little? it might not be totaly out of gear.

BBsGarage · 08-06-2008, 01:44 PM

What is the idle after it has warmed up?

95badbird · 08-06-2008, 01:50 PM

My wifes 02 Taurus does that as well....it seems as if speed and RPMs are in direct corelation with each other.
The more I slow down, the RPMS drop......

FastPlastic · 08-06-2008, 02:52 PM

My Jeep Cherokee works the same way. Makes the N coasting basically worthless on the Jeep. especially since it has the fuel cut off. The only difference I see is I can coast just a little bit farther. The only time I use N is when I'm sitting at a light. Drops the fuel use from 0.74gph to 0.60gph.

wagonman76 · 08-06-2008, 10:52 PM

Within a couple seconds mine drops to 800-1000 rpm when going to N at any speed. Dont have my mpguino done yet but Im sure Im using less fuel in N as well.

Doofus McFancypants · 08-07-2008, 12:25 PM

Next time i am in it i will check a few things.
warmed up idle ( Stoped in D)
warmed up idle ( Stoped in N)
Coasting in gear ( at some MPH)
Coasting in N ( at same MPH)

i will check the GPH and the RPM at those cases.
with that info I will hit the Crusier forums and see if this is common or an error

thanks for the inputs

steve

nascarnation · 08-07-2008, 09:09 PM

A lot of this depends on the gear and control layout of the transmission.
Because there potentiall could be a big difference between the engine speed (low)and the trans output speed (high), and what gear range is selected from neutral to re-engagement.
Some control strategies will hold the engine above idle in a neutral coast to reduce trans load on re-engagement.