Anybody on the board old enough to remember the Mobilgas Economy Run? The techniques and tricks used were very interesting. A great deal of success had to do with looking ahead, feather foot and inertia management. As I paid for my recent tank of diesel, I wondered at just what RPM the Cummins was most fuel efficient. Maybe some of you techno-whizs can come up with an answer for me.
Criteria: Standard temp, humidity, sea level, no wind and flat terrain. Newer series Cummings in 2500 crew cab 4 x 4 pickup. OD auto, 3:73 gears and standard tires.
I'm inclined to think that just lugging along in 5 th gear is most economical, but sometimes torque curves, timing at low RPM and so on will really change the picture.
Looking forward to your ideas.

 

at ~1500-1600 the torque of the engine kicks in, anything below that and you are lugging the engine below that point you are realy burning fuel needlessly, above that point is where the engine becomes more economical. However there is a point of diminishing returns when it comes to higher RPMs. my truck, NV5600(6spd) 4x4, 4.10's. at 70mph I'm turning right at 2300rpm. 2000rpm equals 62mph. From what I have seen from my truck is this. the lower the engine RPM's the better the mileage, this is the same for every vehicle, however the best mileage is not going to be 1600rpm where the torque curve come in, it is actualy somewhere around 18-1900rpm.

Yet I have to throw a monkey wrench into the works. starting in 04.5 with the introduction of the '600' engine, and the intorduction of the Catalytic converter there was a third injection event done by the injectors. this third even is there to mainly keep the cat hot and burn any unburned fuel. 03 and 04.4 trucks did not have this third injection event. The absence of this injection does equal better mileage, 03's and early 04's could easily achieve 20-23mpg. 04.5's and on only under the best conditions could see this type of mileage. Now this third injection event can be turned off, and people developing the programs for the downlaoders can do this, however the engine doesnt seem to run right even if they use parameters of 03 and 04.4 engines. there was just some sort of design difference between the 555's and the 600's.

if you want better mileage, keep your foot out of it, get a towing box or a module and keep it on the lower settings. these boxes on the lower settigns adjust timeing and duration of the injector fireings, give a little better fuel mileage plus a bit more power. I also remember seeing where someone played aorund with timeings and broke down where the best power, best fuel economy, and best of both happened. granted the best power didnt get the best economy, and the best economy didnt get the best power.

 

I'm on my second 3rd Gen CTD, and I can tell you from experience that the mileage logarithmically decreases with increases in RPMs above a particular point. Some of this is due to the engine, some due to the tranny, and some due to the physics of the body and subsequent airflow - and then some due to twenty other reasons.
I put 60+K on my first CTD primarily up and down I-5 between northern and southern California - same road (and no load) week after week...and what I could see playing around as I made that 600+ mile drive was that there were significant drops in mileage when I'd go above 55-60mph and 1800 or so rpms. It wasn't very scientific - but I'd set the cruise at different speeds and run that for an hour on the flat checking the overhead for mileage, then readjust the cruise a little faster and run that for an hour resetting the overhead, and so on. Eventually I had a pretty good sense that to get the optimal fuel mileage at a speed that kept me from getting mowed over by a semi, I needed to keep it under two grand. Above that, the mileage really dropped off.
Now, even with a different CTD, different ratio, different tire size, I still see essentially the same - just going a hundred rpms over 2K and the mileage really starts dropping.
So 2K is my rule of thumb - of course, if I'm not worried about getting run over I will run it about 1600-1700 and get even better mileage - but I don't usually have the patience to go that slow on the interstate!
So even though the actual mileage on the overhead may not be accurate, it's rate of change is pretty accurate and if you can find some long flat straights you can do a little varying tests yourself and find out at what rpm and speed the optimal point is.

 

Thank you gentlemen for your responses.
I'm wondering at just what price for fuel it will take before I force myself to back off running 75 or more! Hard to change.