cummins running at idle, 1000 RPMs in 3rd & 4th gear
 

I work on a military base. This requires driving a mile or two at very slow speeds. 15MPH and 25MPH.

I found that in 4th gear (NV 5600 6 speed) the truck will maintain 15 MPH fairly precisely with my foot off the pedal. This is convenient becasue they write tickets for going 18. But is it an efficient way to move the truck?

It's generally not considered a good idea to lug the engine under a load but in this case the computer is really just maintaining idle speed.

Diesel_dave? SLowmover?

I don't know for your particular case, but in my gasser TSX, idling in top gear is much more efficient than using a lower gear and giving it some gas. Generally speaking, being in the tallest gear possible to maintain your speed is the most efficient way to go. This should be no exception.

My Cummins will idle at 900RPM and hit 25mph. It's an automatic, and I don't know what gear it chooses to putt around in.

Gotta have to agree with that. Anyway, since many truckers still leave their rigs idling all-night long to keep the air-conditioneer working, and construction workers still also often leave theirs idling at the job sites, it should be presumed to not impose any damage to your engine.

This may be a Cummins-specific question, The computer always provides enough fuel to keep it from stalling.

It idles at 700 RPM but when i am rolling along in 4th gear with my foot off the throttle it is turning around 1000 rpms. So it's burniong more fuel that it would idling at a stop.

The question is, does it burn more or less fuel than it would in a lower gear at a proper RPM?

Peak efficiency for this engine is around 1600 (peak torque)

BSFC, 'peak efficiency' is the point at which the engine makes the most torque per unit of fuel.

That's not to say the engine never uses less fuel than at BSFC.

That is to say, if you don't need the amount of torque you'd get at BSFC, you'll use less fuel lower in the rev band.

Sounds like you should do some testing!! I don't know the details of the truck, but can you get some instrumentation to do some testing?

SInce I posted this i did a little experimeing.

THe overhed MPG readout is notoriously inaccurate, but I think it provides relitive information.

When i reset it while idling along in 4th gear at 15 mpg it spiked up to 30mpg so I'm thinking that it's pretty efficient.

Yeah, the overhead isn't going to do much for testing.

What base are you on?? I am at NAS Oceana...

I think the overhad does a lot to provide relative data. when I reset it at 60 on a flat road it levels out around 26

So I can compare one condition to another.

I'm at USCG Elizabeth City