I just checked our fuel logs, I have averaged 15.44 mpg for the last 100k miles, towing, freeway and city driving, I am a bit of a leadfoot, hard not to be when you have so much torque available. Best was nearly 22mpg on a run to Tulsa with 4 guys for the Chili-Bowl Nationals cruise at 78 the whole way. LBZ with 17" rims. 225k total miles so far

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I have an o7 Classic LBZ Crew single rear 3500 every option except sun roof. Close to 100 k now. running 305-55 20s. Check mileage every fill up. VERY heavy foot. Bully dog tune, exhaust, I was getting 13 Non towing for the first 75K or so now the last 25 or so getting a little over 14 go figure. Great truck,
17??? come on guys- Sounds like Granny is drafting behind a big rig lol
Thought about a new Denali 3500 then I saw price!! Kept Chevy and got THIS for half of a new truck^^^^^^^
She gets 9 Period Towing or Empty

With the 5 speed, you should be at 2,000 RPM's at 65 or close to it....with the 6 speed in dads 08 you can run 75 @ about 1800-1900 RPM's. Absolutely love the 6 speed over the 5, doesnt shift too much IMO, but its also running an air intake/down pipe back exhaust, tune, blocked EGR.

Sorry, I have a REALLY hard time believing these guys getting mileage way over 15 with their 5 speed autos. Had one of those before, I know what the RPM vs speed is/was and what the mileage was as well.

I have a hard time believing it too, especsially since I REBUILD transmissions for a living BUT I have experienced it. My 08 get .2 mpg better to about the same in 6th vs 5th towing, 1-2 mpg better at high speed than when I drop it in 5th BUT 2-4 mpg less empty than either of my older 5 speed trucks did and this is after tearing all the dpf and crap off the 08, it was even worse stock.

I'll correct myself. 70 mph at 2100 RPMs. 16 mpg hand calculated yesterday on a highway and local driving combined 285 mile trip. I see 17 occasionally. 19 once on a short highway trip.

16 mpg is what I typically see.

Tire size has been corrected with the programmer. I have no reason to make up my numbers.

I reset my trip every tank and check my mpg.

Borrowed my father in laws 04 CC shortbox 4x4 to pull my 280 to the shootout in August.

it got 10mpg. running 65-75 the whole way down, and back.

My FIL, said it gets 10 MPG pulling his 26ft enclosed trailer/drag car too.

I can say right now my best hand calculated MPG is 23.9 in my 2010 dually 4x4. Fully deleted with Eli live. I have zero reason to lie about these numbers. This was done going about 60-65mph on 100% highway. Realistically, I get 19-20 on all highway trips. I get 12.5 pulling the 43 Donzi from Punta Gorda, FL to Indianapolis, IN

Ditto on the no reason to lie.....2011 GMC Denali Dually. Black Maxx running on HOT setting since 30k. Mbrp 5" straight pipe, DPF delete, injection disabled, cold air intake. Now I have 165K. I can tell you that the wind drastically reduces my mileage, but on a calm day cruising the interstate empty at 75-80mph, I consistently get 20-21mpg hand calculated, and can get 23-24 if I slow it down to 65. Running stock 235/80/17 michelins, 2000rpm at 80 mph. Around town average is about 16, towing anywhere from 10-14 depending on what I'm towing. I don't know why it's so good, but I'm not complaining and I'm not selling anytime soon. Gonna drive it til at least 300K maybe more.

Little off topic but I have a 2004 Dodge 5.9HO 4x4 quad cab that got 17.6 around town for the first 125k miles. At about 126-127k it started getting 18.4. I guess it took 125k to loosen up the motor.. It also gets 20 to 21 soft footing it on long distance highway runs if I keep it under 72mph or so. At 74-75 mph it drops to like 18.

Chris

Some of these posts are spot on, some are comical so it leads me to believe one of three things:

1. We need to get the Guinness Book of World Records on hand to document these amazing accomplishments
2. We need the EPA and the Duramax engineers to see why there could be a 10 mpg difference on similar trucks
3. Some guys can't do math too well!