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2021 GMC 6.6L Duramax 2500HD Trany Issues

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45K views 45 replies 21 participants last post by  blythkd1  
#1 ·
Severe gas mileage issues. My 2019 with same 6.6L engine got 26mpg hwy 19 mpg cityand had 8 speed trans. Brand new 2021 2500hd diesel gets 16mpg hwy and 13 city with the 10 speed Alision. Seems like it doesn't go over 6th gear. I tested with rpm gauge at 60 mph on flat highway in Drive. Dropped shifter to manual and it's in 6th gear not 9 or ten. Also downshifts at speed when foot comes off throttle. Dealership today put on computer 45 minutes, No Codes. Hopefully someone can respond with help. 2800 miles and only trailered a bass boat once at 14 mph hwy. Ugh
 
#2 ·
Kindly note that your 2019 6.6L Duramax did not come with an 8-speed transmission. How do I know this ? If you look at my signature you will see that I have a 2019 Duramax. All 2006 to 2019 Duramax light duty pickup trucks came with 6-speed Allison transmissions ... including my 2019.

Moving on ...

My 2019 averaged 14 mpg city and 17 mpg highway with the factory tires. With my lift kit and 35 inch tires I now average 12 mpg city and 15 mpg highway. The only reason I get that high of fuel mileage with a lift and bigger tires is thanks to re-gearing my axles to a 4.30 ratio. Loaded with camping gear and towing our travel trailer I can get as low as 8 or 9 mpg on the highway thanks to a strong headwind. Honestly, part of it is my fault because I drive 80 mph on the highway pretty much all the time.

Also remember your fuel mileage will increase as you gain more total miles on your truck. I remember my truck got lower fuel mileage when new and slowly increased with time. So my advice ... give it some time and enjoy that new 10-speed Allison transmission. My father has a 2020 and that new transmission shifts so much smoother than my 2019. Also ask yourself the question ... do I view the glass as half full or half empty ? 😉
 
#6 ·
I'm not sure what your refrencing about a 2006. Its a brand new 2021

Uhhhhhhhhhh....... put down the pipe?

View attachment 1084099
asswipe
:ROFLMAO: :ROFLMAO: :ROFLMAO:

Ok, I'll try to help as well. First, UT is right, your 2019 had a 6 speed.

With your new truck, are you driving around in tow/haul mode when you're not towing, or with the engine brake on? What you're describing, sounds like you are.
both are off and tested on computer at dealership
 
#4 ·
Uhhhhhhhhhh....... put down the pipe?
:ROFLMAO: :ROFLMAO: :ROFLMAO:

Ok, I'll try to help as well. First, UT is right, your 2019 had a 6 speed.

With your new truck, are you driving around in tow/haul mode when you're not towing, or with the engine brake on? What you're describing, sounds like you are.
 
#7 ·
Very weird. I guess just push the issue with the dealer. Take them for a ride at highway speed. They should be able to hook up their scanner and tell exactly what hear the transmission is in.
 
#9 ·
Did they drive it with the computer connected?
If so, did the computer show that it was using high gears?

When you switch to M mode, can you detect that it is downshifting?

Usual GM programming includes a downshift when switched to M mode. If in 9 or 10 before the shift, it's not going to end up in M9 or M10 when switched to M mode. (the resulting gear will depend on current gear when switched, and if T/H is active at the time)
 
#14 ·
Did they drive it with the computer connected?
If so, did the computer show that it was using high gears?

When you switch to M mode, can you detect that it is downshifting?

Usual GM programming includes a downshift when switched to M mode. If in 9 or 10 before the shift, it's not going to end up in M9 or M10 when switched to M mode. (the resulting gear will depend on current gear when switched, and if T/H is active at the time)
Just tested with computer driving. No codes and got 13 mpg flat highway at 60. GMC tech putting call into GM
 
#10 ·
I wish GM would've done like Ford and let us know what gear the trans is in. Even my dad's 2013 powerstroke has the numbers 1-6 in LEDs on the dash, so you always know what gear you're in.

I know when I kick it into Manual mode on my truck, it goes to "6" and I have to tap the +/- to go to a different gear, but this is only the max gear the transmission is allowed to go into - not the actual gear the truck is in.

Even if you tap the (+) until "10" is in the display, you have no idea if its actually in 10th gear. My truck shows "10" even when at a dead stop in manual.

Some things in life you are just not meant to know - among them is the actual gear the 10 speed "allison" is in while driving. Unfortunately.
 
#17 ·
Get the Banks iDash ... it’s life changing ... provides all the information GM thought was unnecessary.
 
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#12 ·
My 2020 2500 gets 15mpg around town and if I keep it below 65mph on the highway, I can get about 22-23mpg...

I had a deleted 2010 with an economy tune and never saw better than 22 on the highway, even at 55mph...

If you were really getting 26mpg, you should have kept it, that truck was a unicorn!!!
 
#13 ·
IIRC most all automatics didn't display what gear they were in cause it didn't matter. It was pretty obvious on the old pwr glides but a 400/700/4le/, Fords or Chryslers just said "D" with no manual selection except low.
Why do you have to know what gear you're in? You can't change it like a manual trans so what difference does it make?
Just wondering- no disrespect meant
 
#15 ·
If the took it up to 60 on the flat with the computer hooked up, it would seem likely that they checked what gear it got into.

Curious to hear more about this. Certainly sounds like it has an issue.
 
#19 ·
It's the nature of the Ford 10spd "Allison branded in Name Only" transmission. TFLTrucks noted that the L5P went from the best mileage of the big three in 2019 to the worst MPG in the 2020 model year. The only change was replacing the 6spd Allison with the 10spd 10L1000 Ford trans.

Too bad to. Allison had a 9spd set to go into the 2020 model year...
 
#20 ·
There are write-ups for the 3 transmissions:

The Ford HD 10 speed 10R140, is not the same as the 10 speed in the 1500's and 150's.

Ford 10R140 TorqShift 10 Speed Automatic Transmsision Specs & Ratios


The GM HD 10 speed 10L1000,is not the same as the Ford 10R140 and not the same as the 10 speed in the 1500's and 150's.

GM/Allison 10L1000 10 Speed Automatic Transmission Specs & Ratios


The co-developed transmission used in the LD pickups is GM 10L80 and Ford 10R80

 
#21 · (Edited)
It's the same trans as far as function and size but they are not interchangeable for brand identity and they do have different torque converters

Where's the cut-away pic of the GM 10 speed? Why does the Ford specs identify 18qt capacity but the GM version has the fluid capacity missing? Why is the weight of the GM missing?

Hopefully Mike L. will chime in with his expertise since I am paraphrasing his post from another forum...
 
#23 ·
Well guys. UPDATE. After 25 minute drive with Diesel tech and computer hooked up, no changes 13.2 mpg flat highway, tech was like that ain't right. Unhooked computer and left dealership. Drove 17 miles on relatively flat highway at 60 mpg, note 8 miles was the same we tested on. Bingo 27.2 mpg. And 19.6 mpg around town. Tech can't believe it. Same thing today. WTF.
 
#28 ·
According to the DIC I got 27 mpg on a short trip Monday. I have only hand calculated my mileage once and it was 1.5 mpg higher than the DIC. My 2021 3500 only has 3500 miles on it and I usually get 22-23 on the highway and 14-15 around town. I have noticed the DEF recharge absolutely destroys mileage. I am taking the same short trip tomorrow and I will check the mileage again.
 
#29 ·
Please do. The only time I had driven mine I got 14 mpg highway. Maybe it was getting poor milage because of the faulty def heater? Now that it’s replaced maybe I will get better. Or maybe it needs to break in. 600 miles only. First time buying a new vehicle so I’m definitely going through a learning curve
 
#38 ·
Take your time , lots of info to digest.
LOL
 
#41 ·
Have a 2021 Chevy Silverado 2500HD w/ 6.6L Diesel (6500 total mileage) with the same low mileage problems. Under 19 mpg highway driving. Turbo does not seem to work at lower rmp (under 1800). In two wheel drive (not auto or trailer mode). Diesel Brake not set on and the Cruise Control is off. Still feels like the turbo is trying to slow the vehicle down when going down hill. Best MPG was 26.2 over a 55 mile with a loss of over 5,000 feet in elevation. Dealership said there was nothing wrong?
 
#46 ·
19 mpg highway driving.
Dealership said there was nothing wrong?
I agree with the dealership. Careful believing all the posts claiming mid-20's. It ain't happening. It's mostly just guys seeing a big number on the DIC and it makes them feel good so they don't question it, just report it.

26.2 MPG driving down hill for 55 miles with an drop in elivation of over 5000 feet is not good. It is very poor!
No.....it's not. That's 1.7% average downhill grade. I've came back down long grades like that many, many times and while I agree you should get better mileage coming down that grade than going back up, all other things being equal, it's not worth several mpg.

What were you expecting?
 
#43 ·
The difference on the highway between running at 65mph and 75mph is about 5mpg for my 2020.
if I get real heavy on the pedal and set the cruise to 80, my mpg will drop to about 17. At a steady 65mph I can get about 24 miles.

My new commute to work for the past month has been a steady 53-56mph (cruise control, almost no lights) for 13 miles each way. My last full tank of fuel averaged out to 25.5mpg. That includes the couple lights and stop signs I hit before I get to the main road.