6.6L Gas Transmissions: 6l90 vs. 10L1000

Coop8202 · February 11

Hey guys,

Looking at getting a 6.6 gasser. 2020-2023 offer the 6L90 and 2024-2025 have the "Allison" but not Allison 10 speed. I don't have to have the latest and greatest or the fastest, just want something reliable. I had a 1500 with the 6L80 and had the typical converter issues around 100k miles. Do not want to see anything like that in the 6L90. The 10 speeds seem to have their quirks, and aren't even a true Allison, which is a disappointment. Basically, a 6 speed that has been around for years (6L90) or a newer updated trans that has not been out long enough to show reliability (10L1000).

Thoughts?

Another JR · February 11

1 hour ago, Coop8202 said:

Hey guys,

Looking at getting a 6.6 gasser. 2020-2023 offer the 6L90 and 2024-2025 have the "Allison" but not Allison 10 speed. I don't have to have the latest and greatest or the fastest, just want something reliable. I had a 1500 with the 6L80 and had the typical converter issues around 100k miles. Do not want to see anything like that in the 6L90. The 10 speeds seem to have their quirks, and aren't even a true Allison, which is a disappointment. Basically, a 6 speed that has been around for years (6L90) or a newer updated trans that has not been out long enough to show reliability (10L1000).

Thoughts?

I am totally happy to have the 6L90 on my 2021 3500 hd. It runs great, and has a reputation for being one of the most reliable transmissions available. I could rave more about how it operates, but won’t unless you have questions.

Chevyfan3 · February 12

6l90 has its quirks to and its fair share of convertor issues as well although less common on the 20-23 trucks so far. 10spd the two main issues I see is the front case leak (I had this one) and the valve body issues. Drive a 6l90 truck and then a 10l truck and see which you prefer, that's what it really boils down to.

MORSNO · February 12

My 6L90 has been perfect so far in my 2012 GMC 2500 6.0 with most of the miles while hauling a 3,200lb camper all over Alaska, Canada, and the western US.

Edited February 12 by MORSNO

AndrewF · February 12

I have a gas '24 2500 with the 10L1000. So far I like it a lot, keeps the rpms in a good range when towing and I haven't noticed it being "lost" or jumping around searching for the right gear. As was mentioned it is probably a matter of personal preference, issues can happen with both and neither is vastly superior to the other.

Now if it were a real Allison like the pre-2020 1000 then it would be a no brainer.

Pryme · February 17

I have the 6L90. Not a single issue. I’ve seen lots of negative stuff out there with the 10spd. I personally wouldn’t switch to the 10spd because the 6 has been proven for years and I don’t see a reason. I mean the 10spd doesn’t even rev any lower on the freeway which is crazy to think so it doesn’t get you any better mileage. That at least would be a bonus but nope.

lineman1234 · February 17

I love the 10 speed, i never drove the 6, but my last pickup an HD diesel with a 6 speed was just fine, but the 10 speed is better for ( my ) towing. It shifts smooth and as needed. No problems at all, no leaks at all. ( i do wish those, would quit pissing and moaning, and posting about the little noise the 10 speed makes in lower gears. ) It/the noise is the nature of the beast.

Once a person comes to grips/reality, that a HD gasser is made to work, and fuel economy isnt its strong point, its easier to just except the fact that it will only get so much mpg's. My idea of work,,, is doing what i/a person needs and wants without having to go diesel.

I am retired, and not in a hurry. Going 82mph in a 80 zone interstate, empty, with a headwind that is enough to turn the wind mills nicely, is not good for fuel economy on a 3500 4x4 gasser on winter fuel. Or summer, but summer is a taste better. 55 and 60 paved highways it just puuurs along, still using gas more than a half ton but at the ready to do double or more the work of a half ton.

BuckWallace · February 17

1 hour ago, lineman1234 said:

i do wish those, would quit pissing and moaning, and posting about the little noise the 10 speed makes in lower gears

In case anyone reading this thread is wondering what he's referring to, the 10 speed makes a whining noise around gears 3-6.

I think people (such as myself) were just wondering if that little noise is normal—not sure I've seen any pissing or moaning.

Ry Guy · February 17

I have not noticed any noise coming from the 10 speed.

If you just want a known quantity, something simple and reliable, then just go with the 6 speed and enjoy life. The 10 speed will probably get you more performance out of the 6.6 gas (e.g., for towing heavy), but if you don't need it then just go with the ol' reliable.

redwngr · February 17

On 2/11/2025 at 4:09 PM, Coop8202 said:

Hey guys,

Looking at getting a 6.6 gasser. 2020-2023 offer the 6L90 and 2024-2025 have the "Allison" but not Allison 10 speed. I don't have to have the latest and greatest or the fastest, just want something reliable. I had a 1500 with the 6L80 and had the typical converter issues around 100k miles. Do not want to see anything like that in the 6L90. The 10 speeds seem to have their quirks, and aren't even a true Allison, which is a disappointment. Basically, a 6 speed that has been around for years (6L90) or a newer updated trans that has not been out long enough to show reliability (10L1000).

Thoughts?

2024 - 2025 6.6 gas uses a 10L1000 'allison' (10 speed).

As noted, prior to 2024 the 6.6 gas used the 6L90

6.6 gas vs diesel use a different rpo codes, presumably because of different tcm module to suit the different power curve of 6.6 gas vs 6.6 diesel, and the different diff ratio used for the gas and diesel.

6.6 gas used 3.73's with the 10L1000

6.6 diesel uses 3.42's with the 10L1000

Chevyfan3 · February 25

On 2/12/2025 at 10:20 AM, MORSNO said:

My 6L90 has been perfect so far in my 2012 GMC 2500 6.0 with most of the miles while hauling a 3,200lb camper all over Alaska, Canada, and the western US.

Your in the sweet spot for 6l90...13' and previous were good units. 14-19 when the body style changed we're trash, convertors were garbage and failed taking out the whole transmission. 20-up Gm seems to of changed the convertors because you don't hear about it anymore really. I had a 16' with the 6l90 and it was terrible, finally let go at 55k and that's was with 10k fluid changes. Just a bad design that gm let linger.

dquest · February 26

Had a 2017 2500 HD with 4.10 rear end. Hated towing a 30ft TT with it, hunted for proper gear all the time. Currently 2022 2500HD LT 3.73 rear end. Pulls the same TT with zero hunting around. Average 12 MPG since new ( 53000 KM ) . Routinely pull a 10k gooseneck horse trailer and 14k bumper pull hay trailer.

Drives like a car when empty.

Zero experience with the 10 spd but I am very happy with the 6 sp

Chevyfan3 · February 27

11 hours ago, dquest said:

Had a 2017 2500 HD with 4.10 rear end. Hated towing a 30ft TT with it, hunted for proper gear all the time. Currently 2022 2500HD LT 3.73 rear end. Pulls the same TT with zero hunting around. Average 12 MPG since new ( 53000 KM ) . Routinely pull a 10k gooseneck horse trailer and 14k bumper pull hay trailer.

Drives like a car when empty.

Zero experience with the 10 spd but I am very happy with the 6 sp

That was my experience with my 16' 6.0 with the 4.10 rear as well. I think part of it was because the 6.0 was underpowered and just lacked the torque when you needed it. That truck made for a very "busy" towing experience.