Detroit 12.7L MPG.

I would agree.. to a point. But, higher RPM's do not always equate to poor mpg. There are too many variables. Terrain, load, weather all have a play in this. Low RPM rotational inertia thresholds, EGT's, turbo efficiencies and a hundred other things go into the factoring.

It is true, that under a pull, if one is at a higher RPM, the GPH will be higher. But that does not tell the whole story. On a pull, I will be using a higher GPH at 1500 than running lower, but I also do not have to downshift on the hill and I crest the hill sooner, the actual fuel used may be less for the entire pull than if I took longer to top the hill by running at a lower RPM and downshifting. EGT's will generally be less, showing that exhaust flow restriction is low and efficient, there will be no loss by the engine overcoming low RPM rotational inertia, etc.

The charts, be it for general fleet use or the "conscientious" driver, are general guidelines based on ideal operating conditions and do not factor in terrain, load, or weather. Low RPM running has it's place, but is not always the best solution. It is very impractical when hauling 46,000 lb of oat meal or honey out of Sioux City on 2 lane highway 20 before the flat land area further east. Or at the other end, from Dubuque to Stockton, IL on that 2 lane stretch. Or even the 4 lane of highway 151 from Dubuque to Madison, WI.

And the biggie that one can control regarding mpg, is allowing the truck to do the work and not the engine. When cresting a hill, let the momentum of the truck carry the truck over the crest and let the down hill give you back the road speed. That is probably the biggest failure of folks to understand, and the major downfall of staying on cruise control all the time. If there ever was a principle to efficient running, this one probably would give the best results.

Life would be so much simpler if we lived in a perfect world. But I was comfortable, running 1400 rpm all day, with roughly a 10 mph south crosswind, and getting 8.2 mpg for the day as per my fueling last night. Nice 4 lane running, traffic not bad, and gentle rolling hills. If every day's running was this way.

 

Are the inside of your fuel tanks black as well?? If so you may have a fuel problem asphaltine or algae ... also if fuel lines are original the rubber lines could be partially collapsing internally and look good outside

 

Hey Cowpie
Any idea Cat C15 accert 475 HP 13 speed 3.36 rears best rpm range for the optimum efficiency would be ?I use the cruise a lot just for the comfort,also thinking that I am saving fuel.Runing mostly 64Mph.1170 Rpm,getting average 6.25 Gpm pulling flatbed.Thanks

 

2002 should only have a waste gate turbo on a 12.7 pre egr Detroit

 

You may need to get your injectors, injector seating, and possibly head and rings checked out. That blackness is likely coming from exhaust gasses finding their way into the fuel.

Also, rubber lines are only reliable for 2-5 years. Any longer than that, and you're begging for a failure.

 

That won't show anything. The turbo doesn't kick in until your under way or with a load.

I have pretty much the same engine and specs as you except I'm at 500hp. My best fuel economy is in 9th gear at 57mph. Shoots way up!

 

And by the way. A spray bottle of Simple Green is great for finding leaks. You can spray or steam to get into hard places. Bubbles up nice.