Fuel Mileage Puzzle - 06 5.9l CTD

[Karter]

I'm attempting to get my arms around exactly what kind of fuel mileage I'm getting with my new-to-me 06 2500QC. I know this sounds ridiculously simple, but for some reason, my calculations are all over the map.

Since I just recently acquired this truck, I really don't know what, if any, mods have been done, other than the obvious ones. It would appear that the engine is stock with about 140k. The truck has about a 3" lift with 285 70R 17 tires. According to the on board fuel economy display, I'm getting around 19mpg on strictly hwy driving and around 16mpg on mixed city/hwy. Towing a 7x14 enclosed trailer, more like 12mpg. If I hand calculate the mileage using the odometer, it seems like several mpg less.

I don't know what the stock tire size is, but I know the speedometer is a couple mph slow, based on a couple of gps comparisons. When I try to verify the odometer to the mile markers on the highway, it's always about 7 tenths of a mile less than the mile markers over a 20 mile stretch.

What are other people getting for comparison? Is my tire size affecting the calculation? How does the computer calculate it?...and over what time frame? And am I doing something wrong when figuring it?

Thanks in advance.

 

[B.G. Smith]

I have the 06 3500 and it has 235/80R17 tires and that is the size listed on the door panel. The tire size difference might be the error in your odometer. The mileages you list from the fuel economy display are very close to what mine usually shows. My real world mileage is 16 to 18 unloaded and about 9 or 10 pulling a 31 foot fifth wheel. Even a small two wheel trailer wit two four wheelers on it will pull the mileage down considerably. As for calculating mileage between fill ups, it will vary some because it is virtually impossible to get the same amount of fuel in the tank each time. If you used the same pump and the same side of the pump every time it might be close. The tank is listed to hold 35 gal. on mine and the 2500 may be a little less but if you could get all the air out at fillup I think my tank would hold near 40 galons. The fuel mileage is somewhat of a novelty, it is my understanding that the computer takes aan input from the engine rpms and the accelerator position to calculate the mileage. My highway reading are always 2 to 4 miles high and the towing is 1 to 2 miles high. bg

 

[spin05]

I get 14+ unloaded and 12 loaded on the overhead. Hand calculations are about 2 mpg less. Copied this from the facts/technical section:
Q What mileage can I expect from my Turbo Diesel truck?
A Regardless of the year/model of the truck, you can expect:
12 - 18 mpg – without a load
9 - 13 mpg – with a load

My truck is a 04.5 thou........

 

[mfurrh55]

My 07 5.9 gets around 17-18 unloaded and 11-14 depending on load, really heavy gets 11 light 14, my 01 i used to own got 21 empty and 10 with heavy load, 16 light load

 

[campertruck]

I have an 06 3500 SRW automatic. 18mpg empty , highest was 20 mpg driving 50mph straight.
With 11'6" Lance camper on the back towing 20 ft trailer at 7500 lbs I get 13-14mpg.
No difference with or without trailer behind camper.
The mileage display is always more optimistic than hand calculated.
I am running factory sized 17" tires, no lift , no mods.

 

[AWeiman]

our 06 gets between 13-16. its empty all the time. our overhead is always 4mpg higher than actual.

 

[p-Bar]

I top mine off all the time to get accurate reading . You can put 40 gallons in to the neck I average about 15 with 500lbs tools in back .

 

[cerberusiam]

Is my tire size affecting the calculation?

Yes, big time if you have not corrected the odometer to tire size. The Lie-O-Meter on the over head does a calculation based on observed fuel consumption thta is purely driven by inteprolated readings, it may or may not be close to correct. Most of the time not even close.

If you want accurate numbers you will need to play with a tuner capable of adjusting the odometer and a GPS to get it as close as possible.