Home Forums Hunting & Shooting Hunter's Campfire GMC 6.6 Duramax mileage?

I had a 2005 GMC 2500 LLY that I put a chip and upgraded exhaust, it got 23-25mpg on highway and 18 in town. I now drive a stock 2021 GMC 3500 and get 16 in town and 21 on highway. Highway is unloaded mileage.

15.2 on the gage .. that tells u all your ilde and run time... 19.8 if I run to Anchorage.. ..I run 85 miles a day 4 days a week...2004.5 Duramax twice a week 200 gallons of water....

I’ve got two, a 07 and a 06. Both get 18mpg hand calculated over the past 5 years. Fuel economy is better in the summer and worse in the winter, but that’s the average over a mix of hwy and city. They are great trucks.

Mine's a 2016 3500 and that's exactly where I'm at.

I had a 2011 stock that would get around 14.5 for mixed driving, maybe 17-18 on the highway, and 10-11 towing a 6k travel trailer.

I’m a Cummins guy, been driving current goat since 02. But wanted to say, this is the most completely honest sounding, totally believable mileage post I’ve ever read, hat off to y’all.
Usually when diesel mileage comes up there’s always “that guy” with unbelievable claims.

Hey Cisco, I am curious. What diesel pickup was out in 75? Ford started production in 84.
I currently have a 19 6.6 and if I behave myself and limit my trips to Montana I can stay right around 22 mpg. I just made a bonzi trip to Montana and back this last Saturday (785 miles) and I only got 20 mpg, but the roads were not good enough to let me travel my normal Montana speeds. When I do that my mileage drops to about 18 mpg. Now with the travel trailer on my milage drops to about 12 mpg.

You are right Blu Mtn, my first was an 85 Ford , not a75. However I have had about 8-9 Duramax Company trucks since then. My current '08 has had the emissions deleted and is chipped. THE ONlY ADVANTAGE TO DOING THAT , is the fact that I won't have to futz around with emissions setting plugged up.

We have gone to gas trucks , I don't drive he diesel too much any more, it is good truck , but diesels are too expensive to buy initially and maintain. When the advantage of diesel prices went away decades ago ....so did the advantage.

I liked diesels and have millions of miles in them , but no more.

Back when Ford introduced the diesel in 84 we were driving 81 one ton 4x4 Chevrolet's with the 454 in them. We were getting about 5 mpg. We bought the Fords and jumped to about 15 mpg and diesel was half the price of gas. It was a win-win for the farm. Through the years I have owned about 20 diesels of either Ford or Chevy and have driven about 10 one ton service trucks of all brands. But with the horsepower and fuel economy that the gas engine are getting now I don't know what my next pickup will be. I do guess I know what it won't be and that is a Dodge.

If the govt allowed a diesel to be a diesel, you guys would be getting much more power and much more mileage.

There's a fix for that in some places after warranty is gone.

Fixing a GM pickup is much more expensive than the same fix on a Ford. But still probably worth the price of admission.

I've got a '18 Denali. Getting 16-18 city and 22-24 hwy empty.

My late build 2006 LBZ would get 16-17 unloaded regularly with our bio-mass deezil fuel. I oddly picked up about 1 mpg after installing twin Kennedy centrifugal lift pumps, and was about to update the brainbox, when a guy made me an offer I couldn't turn down. If the truck would have had a 96" bed instead of 78", I likely would still be driving it.

Blu Mtn ,
I just remembered International was offering , a dealer option perhaps, a Cat diesel in their trucks. I looked at one at a dealer and did not care for the way it was installed. Don't remember the year.

One can buy an awful lot of gasoline ...for the costs one will have with a new diesel. We get awful good mileage with our new Fords.

I went with Ford , as GMC has had horrible QC issues. Ford may be no better. However , with 50K miles ...no problems.

I still have Chevy Suburbans and an old '52 P'up.

I had a Diesel Suburban that my wife drove the hell outta it.
Also, had a 1/2T Suburban with an Olds Converted diesel, GMC/s finest moment,

Could not keep tranny's in it . One well know trans outfit put several in under their warranty.

I probably have more small diesel truck experience than most.

I am pretty well versed in diesels, as I have a track loader, diesel tractors, ran a lot of diesel powered heavy 'chinery from 2 engine scrappers , D 8 dozers to finish compressors.

Some one mentioned "Fixing " the diesels after warranty is up for better mileage.

That is pure un- adulterated BS. I have been involved in some of these conversions and chips . More power no more mileage .
I don't get any better mileage with my Duramax. I do not keep my foot in it either.

What highway speeds? Are you using AT or highway tread type tires? I see folks saying they get over 20MPG w/3/4 and 1Tn diesels, but I've never seen it play out at the pump. In 350K Mi myself between 2 F250's, a 6.0L and 6.7L (w/3:31 gearing) I've never gotten either of them to make 20MPG+ over say a 200+ Mi all highway trip. That said, our interstate speed limit is 75 and I drive 79-80MPH with the cruise set, 75MPH on the non-interstate 4-lanes, and 70MPH on 2-lane highways. Maybe 60MPH and highway tread would get me to 20+? I'm not saying it can't be done, I'm just wondering how others make it happen?


Give or take, +1.5-2MPG on the highway eliminating DPF and EGR with tires and speeds remaining basically same/same.

I looked at that fuel usage screen after reading the post. It says 19.7 mpg. My last long trip, 1200 miles, it gave 24.5 mpg by actual computation. I bought this 2005 DuraMax Silverado last January. It’s a 2500 HD. Has that small back seat and the rear doors that open backwards. Fellow I got it from used it to pull a big camper. He put in some sort of chip. 136,000 miles when I got it, and it is like a new truck.

Don't know about the GM trucks, other than it cost twice as much to "fix" one of them, vs a Ford "fix".

Not sure about additional HP, but I went with a basic computer program that claims over 50hp.

I do know for a fact that my mileage immediately went from 13mpg to 17.5... I'll take that all day long.

My trailer pulling mpg went from about 11mpg to 14.5mpg.

I do not believe any of it !

More HP,.... mileage , perhaps a little better ....if not ripping around.

NO one, that is ,no one ,is gonna say not much diff after putting the money inna new chip and exhaust.

Dude, I don't know you, and you don't know me.

If you did, you'd know I don't BS and lie. There's just no reason to do so.

The numbers I stated are indeed truth as it stands before and after.

You can believe that or not. Frankly, I don't care which.

I used to have to start my Duramax ....with a pony motor.