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Drivetrain - 2011+ Discuss front and rear axle, transfer case, brakes, etc. Transmission questions belong in the appropriate transmission forum below. Fifth generation and above...

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Old 11-04-2019, 01:26 PM   #1 (permalink)
lost2a6
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5th gear vs 6th gear fuel mileage

I just purchased a new to me 2016 GMC 3500 Denali dually LML. I noticed at 70 mph according to the computer, it gets better fuel mileage in 5th than 6th. At 75 mph it's a toss up, however at 80 mph it gets better mileage in 6th. No, I haven't hand calculated it, however the mileage differences are consistent and to me is irrelevant if the computer is off a little from hand calculating. It appears that GM missed the boat with the 3.73 gearing in this truck and it would be better off with 4.11's. I know that my old LB7's sweet spot is around 2,000-2,200 RPM and my new LML seems to be the same. My truck is 100% stock, stock tires, etc with only 52,000 miles. So this brings me to my question, has anyone ever replaced the 3.73 gears with a 4.11 on a stock LML Dually and what was the results? To me this would be a win win situation. It would pull better, accelerate easier, along with being able to run in 6th at the slower speeds and getting good mileage.
Monitors, please move this post to the drivetrain section if needed, thanks.

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Old 11-04-2019, 01:46 PM   #2 (permalink)
OkDually
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Thread moved over to "Drivetrain 2011+"



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Old 11-07-2019, 03:14 PM   #3 (permalink)
D_R_C
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Engine happy RPM’s is part of the mileage.
That’s why your seeing a difference depending on gear and speed.
Compare rpms differences as well.
IMO when towing never let it shift into 6th gear, lock it out by manual shifting.

Going by instant mpg with the DIC is more of a fantasy mpg.
Everytime you reset it the mileage quickly changes depending on the terrain until you have a couple hundred miles since your last reset.
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Old 11-12-2019, 11:48 PM   #4 (permalink)
ktmrfs
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do an average over at least 50 miles or so. I've never seen better unloaded mileage in 5th than 6th on my LML (and I've got 55K miles on it). And in the vast majority of the cases when towing 6th gives better mileage than 5th. And the LML consistently gets about 10% better mileage on long trips unloaded than the 2004.5 LLY did. partly is due to the lower rpm with 6th gear and slightly larger factory tire diameter. In town and towing mileage is pretty much the same on the LML as the LLY

Engine frictional losses are related to the cube of the rpm. doesn't take much of an rpm change to make a difference in frictional losses.
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Old 11-12-2019, 11:58 PM   #5 (permalink)
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There is a risk towing in 6th gear unless you can maintain at least 1700-1800 rpms.
Below that you’ll be lugging the engine, monitoring your ECT, Trans temps and EGT’s will show 6th towing isn’t a good idea.
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Old 11-13-2019, 01:21 AM   #6 (permalink)
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Put your truck in T/H and let it buck. The truck knows what to do. I’m towing in 6th most of the time. There’s no problem with it, it’s quieter and better mileage.

Also, don’t use manual mode when towing, it disables automatic grade braking.
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Old 11-13-2019, 02:28 PM   #7 (permalink)
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Quote:
Originally Posted by BanksLB7Duramax View Post
Put your truck in T/H and let it buck. The truck knows what to do. Iím towing in 6th most of the time. Thereís no problem with it, itís quieter and better mileage.

Also, donít use manual mode when towing, it disables automatic grade braking.
exactly on both comments. about the only time I use manual mode is going uphill on a road with lots of curves that require slowing down for. It avoids constant up and down shifting that.

My experience is that in T/H mode it doesn't go into 6th gear until between 60 and 65mph. So you don't have to worry about dropping the rpm down to low.
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