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My "06" 2500HD 4x4 LLY Duramax has 32,200 miles, Ihave a K&N Filter, I pull a Truck Camper & modified horse trailer. I make long trips when I use it. When I first got it the computer, which I reset religiously, said I was getting around 14 loaded and about the same unloaded. That mileage was for about the first 12k. After that it has said I was getting 12.5-12.7mpg.
I just took a trip, for about 2100 miles & all of a sudden my computer starting saying 9.9-10.0. I started adding about 6k ago Stanadyne fuel conditioner. I find this unacceptable. I am finicky about the maintenance. I am taking into GM next week, but would like this forums input. I am doing the same things, I have been doing since truck was new. I just filled up yesterday & checked what computer says on fuel consumption & was w/in .2 of gallon. I have found computer to very accurate. Please give input!
 

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I would change the fuel filter and go back to the original AC air filter. Check the rubber (blue) sleeves to and from the aftercooler/turbo to insure they have not blown loose. Stanadyne fuel conditioner is GM recommended.
 

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Where do you live. Cold weather has a very negetive impact on fuel mileage. :mad:
I agree with getting rid of the K&N, the filter oil thickens in cold weather.
 

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Get rid of that KN filter immediately would be my best advice. Also, I noticed my mileage dropped with age on my 06 LLY also.

Do you ever hand calculate your milage? Mine is between 11, pulling ~15k lbs over steep mountain grades at 60 mph, to 19ish, the once or twice I"ve gone empty on the highway. Best mileage ever was from the dealer, 20 mpg, and it's gone down hill since.

Give a read to my lots of smoke thread, lots of good information about the negatives of the KN filter on our rigs. http://www.dieselplace.com/forum/showthread.php?t=195326
 

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With the winter fuel additives it will drop. Mine dropped my last trip pulling my travel trailer, so I added some cetane boost and it went right back to 13.5-14, which is what I normally get going 75 - 80 with my trailer. I figure its not worth worrying about mileage....Bigger families= Bigger vehicles.
 
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