Coasting in gear vs. neutral (fuel cut feature)

[busypaws]

It depends on how you setup the scangauge. You can program the scangauge to calculate fuel cut off on coasting. Or you can program scangauge to assume you idle on coasting in gear. How do you know what your fuel injector actually do so you can program your scangauge? Does anyone know a place to look this up or a way of determining. I'll do some searching to see if I find any answers.

[kc7ekk]

I just finished some research on this exact topic. My findings are published here:
kc7ekk-solar.blogspot.com/2011/04/coast-in-neutral-or-in-gear.html

The answer is both.
With a cold engine, coasting in neutral uses 12% less fuel than coasting in gear. (at least for the default values in the computer of my wife's 2002 Chevy Venture). When the engine is warmed up, it flips the other way and coasting in gear saves on average 2.7% more fuel than coasting in neutral.

[kc7ekk]

Originally Posted by busypaws

It depends on how you setup the scangauge. You can program the scangauge to calculate fuel cut off on coasting. Or you can program scangauge to assume you idle on coasting in gear. How do you know what your fuel injector actually do so you can program your scangauge? Does anyone know a place to look this up or a way of determining. I'll do some searching to see if I find any answers.

Before I messed with my scan gauge settings I was getting "9999" as the MPG during times of higher speed coasting. After setting "fuel cutoff" from 24 (default) to 0. My scan gauge was able to report back very high MPG readings without going into overflow mode (9999).

[euromodder]

Originally Posted by kc7ekk

With a cold engine, coasting in neutral uses 12% less fuel than coasting in gear. (at least for the default values in the computer of my wife's 2002 Chevy Venture). When the engine is warmed up, it flips the other way and coasting in gear saves on average 2.7% more fuel than coasting in neutral.

What do you mean with "coasting in gear" ?
Pushing in the clutch and leaving the gear lever in the gear it was in ?

[kc7ekk]

"Coasting in gear", refers to letting off the gas but leaving the clutch in the gear it was in. Preferably a high gear so the RPM of the dead weight of the engine doesn't slow you down too much.

[justjohn]

euromodder is probably going to tell you that DFCO (coasting in gear) is always worse, but I was theorizing about this on the way home from work today and I think it probably depends on how much braking you get from the engine. Some cars lose a lot more energy when in gear than others. I wonder if DFCO can out perform engine on coasting in certain long geared vehicles.

[Arragonis]

Depends on the slope and coast. Coasting uphill (P&G) or on the flat then I slip out of gear and the SG2 shows 0.12 GPH and 300-500 MPG depending on speed.

On a slight downhill I can go into 5th or 6th and coast in gear - the gear chosen is low enough so that engine speed is above idle, so the car is 'turning' the engine and not the other way round. The fuel used drops to 0 GPH and 9999 MPH, and the loss of speed isn't that bad - enough to finish the slope.

On a steeper slope then in gear drops consumption to 0 GPH and MPG to 9999.

[euromodder]

Originally Posted by kc7ekk

"Coasting in gear", refers to letting off the gas but leaving the clutch in the gear it was in. Preferably a high gear so the RPM of the dead weight of the engine doesn't slow you down too much.

The MPG while doing so will be better than with the gearbox in neutral, as the fuel flow is cut off (DFCO).
But the downside is you won't get nearly as far.

Coasting in neutral and with the engine on is what lowered my fuel use considerably over DFCO.
I still underestimate just how far it'll roll


My car is a diesel, does DFCO right down to 1000 rpm, and the engine braking is considerable.
I've always made good use of the DFCO feature - well, before joining ecomodder that was - but it never consistently returned the kind of MPG I'm seeing now.


If you use MPG for recording, it'll bias towards DFCO, as you get towards infinite MPG (division by 0).
This can upset the readings you've taken.

[skyl4rk]

The real issue here is that coasting in gear (engine braking) is far more efficient than braking in neutral.

If you are truly coasting without the need to brake either at the moment or in the near future, coast in neutral. If you see a braking situation ahead, coast in gear.

[tjts1]

While in DFCO I have it setup to automatically turn on the EGR valve and try to equalize the pressure between the intake and exhaust manifolds. This increased the distance I can coast in gear with injectors OFF.