I was averaging mid 16 mpg during the winter months and 18's during the summer. That is running 65mph with 4:10's. I also accelerate with the "egg under my pedal" theory (it works!) Recently I dropped my speed down to 55-57mph which dropped my rpm to about 1750. Now I am averaging around 18 in the winter. These are big trucks with a lot of drag, be thankful to get what we do!

I also tried the 2 stroke oil. It definately quieted my engine down, but I saw no change in mpg, so I went back to power service for the anti gelling properties. Maine can get pretty cold and I can't afford for my truck gel up!

 

I'm not sure where you got that cetane figure. I rarely see it shown on the pump. Doesn't matter anyway, BTUs per gallon determine mileage. ULSD has about 1% fewer BTUs per gallon than LSD.
http://list.priceweber.com/cummins/td/ulsd.html
In '06, the year that ULSD first started appearing at the pumps I used 4687 gallons and averaged 13.03 mpg combined towing and empty. In'07 6873/12.87 and '08 4561/13.7 I don't see any drop there, much less 20%.

I believe you are on the right track on the bio, and I agree that advanced timing in the 15 to 16 range gives better mileage.

 

I have 3.54 gears, and a nv4500. The poor ol'e thang also has 250,000 hard earned miles. But still uses very little oil in a 3,000 mile oil change. I think the timing issue will help with the mileage. As far as i know the truck has never got better than 14 mpg.

 

Do you have gauges? Have you checked for boost leaks, turbo operation, clogged air system? Does it smoke a lot? Have you put the plate back to stock location and checked mileage. Do you do a lot of WOT? Are you burning WMO, ATF or other crap not intended for for a diesel engine? Mileage that bad would have indicators as to why.

 

Cetane is very integral with compression ignition performance
ASTM D975 puts cetane at 40 minimum and that's where our fuel sits. Local Cummins branch periodically tests area fuel sources and it may hit 41 but usually holds in low 40's.
This is a local thing. Other parts of the country enjoy very high cetane fuels, for now. EPA wants to lower cetane.

 

fwi,

At 29mpg you should be able to go 1015 miles before your tank is dry. I'd be happy to see you go 900. Your low fuel light should be just coming on.... Then I'll believe the mileage figure. At 29mpg you would use 20 gallons which would be a little less than half tank, but way more than 1/4. Some solid math would help (miles driven on odometer / full tank - amount put in at destination fillup)....