My dads 05 F350 6.0L/auto has the same problem with fuel mileage. Running empty down the highway he only gets 13 mpg towing 14,000lbs (either his 5th wheel camper or S250 Bobcat on our gooseneck) he get 11.5-12mpg. His truck is back to being bone stock, he had the Edge Juice with Attitude but took it out and I am going to list it on Ebay for him, he is going to get some custom tunes from Innovative Diesel next.

He has had the truck to the dealership 4-5 times about the fuel mileage, they have reflashed the PCM, they have replaced all the filters a couple of times. They even replace a couple a injectors that weren't putting out full capacity.

Dad is very unhappy with the mileage of this truck, especially since his old truck (same exact configuration only a 99 7.3L/6spd) would get 16-17 mpg without a problem and had a high of 20/21 mpg a couple of times, towing he would get 12-13 mpg.

Dad has actually talked about trading this truck in on a GMC/duramax or Dodge/Cummins, since thet seem to be getting alot better mileage.

 

13 is low being empty, but I would like to know how he gets 11.5-12 pulling 14k lbs. I get about 8.5-10mpg pulling my 5th wheel which weighs about 8500lbs loaded. Empty I get anywhere between 13-17 depending on city/highway and which tune I am running.

 

I don't know how he gets that mileage while towing. He said on the way south to Texas this past winter that he averaged 11.4mpg and on his way back north 11.5mpg. If I remember right he said the lowest that he got was 10 or 10.1, when they got into alot of head wind, and a best of 12.7 or 12.8

The only reason that I can think of that he gets that good of mileage towing, is that he only does 60 mph, or 65 if he is really pushing it. Also I figure the reason that he gets alittle beter mileage while towing the gooseneck is because of less drag than the 5th wheel. Another strange thing is that with the pick-up camper on his truck the mileage is basically the same as with the 5th wheel in tow. The frontal area is about the same with the pick-up camper and the 5th wheel, but the 5er weighs about 10000 lbs more.

We have no idea what is going on with the mileage, why its so good towing, but just sucks being empty. And no one else can give us a good answer either, we have post about this a few times on thedieselstop.com, thedieselgarage.com, and I may have even asked on here. Also the diesel master tech at the dealership is stumped about it.

 

I just skimmed through so maybe this has already been said...

Are you compensating for the taller tire when calculating your MPG? You may be getting A LOT better than what it seems depending on your stock tire size.

 

Im not sure what comes on my truck stock... It has 285/70/17s now... I am thinkin 265/70/17?? Not sure that would make a huge difference. I normally calculate it by hand calculator and get worse than what the computer shows...