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I took a trip from Virginia to the Mississippi coast last week. My outfit is a stock '05 CC, SB with a topper pulling a 16' enclosed trailer. My gross weight was 13,700 lbs. Before the trip I turned 3000 miles and serviced my unit. Rotella t 15w40, fuel filter, synthetic in both axles, Allison spin on and tire rotation. Tires were gauged to the max on truck and trailer. Speed was set with the cruse. Mileage was hand calculated and I must say the lieometer was within tenths on the mileage and amount of fuel used. Ten tanks of fuel were burned on this trip. Mileage was also checked with the tow mode on and with the tow mode off. At 75 mph mileage was from 9.94 to 10.31. At 85 mph mileage was from 8.17 to 8.48. Yes I know this was running hard. Ok here is the kicker the other trucks a Ford 6.0, a Ford 7.3 and a new Dodge Cummins each got 4 or more mpg better than mine at each fuel stop. These trucks were also stock. Each of these trucks were pulling enclosed trailers of about the same weight. Mileage was consistant so I don't think winter blend was ever used. Idle time was not an issue. Am I crazy to expect better mileage? Everyone expected my outfit to get the best mileage. Between the poor mileage and the quart size tank my DMAX was sucking hind tit. Is this the best it will get? Should I try the newest flash available? I don't have any of the problems of the earlier models, surging, overheating etc. I go out west to hunt a couple of times a year and will be needing a bigger tank for sure. Any advise is appreciated....
 

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best bet for best mileage is keeping it at or under 2k RPMs, anything over it just goes down very quickly
 

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Can't really say how good a Wester tune will help with a load it sure made a big difference empty drive 69 mph. Got 22mpg with winter blend should really help with a load it will be a while before I can get gooseneck trailer with camper and play truck loaded to check mpg.
 

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My early mpg's were similar to yours

After I turned 1,800 miles on my 2005 DMAX 2500 HD 4x4 CC SB with Apr 05 fabrication date, I drove it on a long trip (2,400 mi) pulling a travel trailer. The trailer body is designed with very good aerodynamics, Trail Lite version by R-Vision. My gross weight must have been similar to yours since my trailer weighs around 6,200 lbs fully loaded and of course I have the 4x4 which adds a few pounds. The bed was full of stuff including three bikes and there were three people including myself in the cab. In that trip, I averaged around 11.5 mpg in Florida and then around 9-10 mpg in Georgia, specially around the mountain areas. Typically I was doing 75 mph. This took place during the summer. As soon as I did some of the cooling mods discussed in this forum, including the cold air intake mod that many of us have bought from a contact in this forum, I noticed improvement on my mpg's around town and while pulling my trailer. I have gone as high as 13 mpg while pulling a TT here in Florida with the mods while driving 75 mph. I have not gone back to the dealer, so I have the same factory flash.

My truck just recently broke the 10,000 mile mark and all I can say is the mpg's are still getting better. I think I am also getting ready to change my fuel filter, it is the original one. And also, I am running synthetic oil and axle fluids. The Ally is still the same factory fluid with new spin on. I keep the pressure on the tires as speced by GM.

All I can say is, make sure your fan clutch is not working over time on you since that will rob your mpg's due to the heating issues caused by improper air flow and give your vehicle some time to break in. Also, having a well designed trailer with aerodynamics helps some. I also have my bed covered.
 

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The first road trip with my '05 Sierra was from MA to Bowling Green KY to pick up a Covette. I then continued to Panama City Beach before heading back to MA. Most of my mileage ranged between the low 9s and low 10s. This was pulling a 24' enclosed car hauler that weighs about 10,000lbs loaded. I was running about 70-80 most of the time (tried to hold about 70). I recently sold that trailer and will be pulling a 32" aluminum enclosed with a V nose. Hopefully this will help with mileage during this year's track trips. I'm also getting close to the magical 10,000 mile number. Alot of people say they mileage improves after that.
 
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