GM 6.6 gasser and 10-speed opinions?

[redhat]

Those of you with newer GM 3/4 ton and up trucks with the 6.6 L8T DI and the 10-speed 10L100 “Allison”.

How do you like it so far?
What’s your MPGs been like?
How does the drivetrain perform? Any weird shifts or anything goofy?

Been looking at newer 3/4 tons, especially looking at a pretty competitively priced regular cab in my area.

 

[MrHorspwer]

I have a 2024 Silverado 2500 with the combo. It was purchased to primarily tow an 8,800 lb travel trailer and handle my three-day-a-week 20 mile round trip commute.

As a powertrain platform, it performs great with the trailer attached. Very confident in gear selection, readily downshifts when needed and upshifts when the load is low, and the engine delivers plenty of power (so much more power than the old 6.0L) While I don’t live in a particularly hilly area, the few downhills I have encounter (as recent as this past weekend) will induce downshifts for additional engine braking on decel. With the trailer disconnected, there is a massive amount of torque management and the throttle pedal buffering is noticeable off the line. I’d imagine a pedal monster or whatever (when they become available) will really wake the engine up. The only weirdness I’ve noticed is a couple rough 2-1 downshifts in the parking deck at work. It seems like if you interrupt the 2-1 downshift with a throttle input, it’ll rush the shift and give a noticeable bump. This is low speed, parking lot type maneuvers.

Fuel economy during my stop-and-go commute isn’t great, 12.5-13.5. Park the cruise at 70 and it’ll do mid-16s, go faster and the number suffers. Best I’ve ever seen is a little over 17. With the trailer on the hitch doing 70, it’ll return 7.5-8.5. Best I’ve seen with the trailer is 12 on a lazy 55mph backroad (I was initially incredulous but it did the same number on the return trip).

 

[JavierH19]

I like it but i think i'd look at the 7.3 gas ford.

 

[cpayne5]

I have a 2024 2500 with this powertrain combo. I tow a ~7500lb toy hauler. What's this MPG thing you mention? Never heard of it.

I've put about 2000 miles on it towing this summer (2500 total miles on the truck). Mostly here in Virginia. Nothing crazy. It initially averaged 7ish, but has risen to nearly 9MPG. It has been exactly what I wanted. I've never owned a diesel, so I don't know any better in that regard. It will occasionally shift weird when I forget to put it in tow/haul when towing.

 

[sohccammer427]

It's interesting that the Ford and GM 10 speed were co developed. From what I have read that transmission is not actually produced by Allison, like the older 6 speed automatic. That being said, I've seen a few videos that seem to indicate that the Ford version has better build features. But I'm now more of GM guy and if I were in the market for a 3/4 ton, I'd not let that influence my decision.

Allison 10L vs Ford 10R Short 4 minute video.

 

[userfriendly]

That engine/transmission combination would be awesome in a Camaro. No annoying stop/start feature, cylinder deactivation or 0W20. They could even call it a 396 like the original 402 BBC.

 

[21_6.7f450]

I've pulled 8k of enclosed trailer with a 6.6 Gas/10spd. Ran decent. Got 17 running to get the trailer and 12 with it hooked up.
Power was fine foe my expectations of a gas rig.
Stay away from Ford. 100% recommend the Chevy.
Transmission aside, I've NEVER seem such poor QC as a Ford superduty from 20-24. It is awful. Rear end issues, missing fasteners all over the truck, tailgate cables of enequal length, sand under clear coat, missing fuses, a bad run of AC compressors on the 23/24's and dealers put back on junk ones....dealer network second from the bottom...One of my rigs

 

[JWC86]

I have a 2020 F350 7.3L 10 Speed with 80k miles.

I recently borrowed a 2024 GM 6.6L gas with the 10 speed for a few days.

No comparison the GM’s trans is WAY smoother than the Ford. The GM is like silk while the Ford is like 1 grit sand paper. I’ve heard Allison took care of tuning these transmissions for GM and they were not going to put their name on it unless it was smoooth, and it is.

However, other than the clunky shifts I like the ford better. The GM seems to prefer to hold gears and rev, it actually drove me kinda nuts. The Ford likes to shift early when just cruising around and that 7.3 has great torque down low to lug it around nicely. Reminds me of my early 2000s diesels more than it does a gas truck.

The 6.6 isn’t a slouch but doesn’t have the stump pulling power of the 7.3 IMO. Feels like a big improvement over the 6.0 however, including not looking under the gas tank while filling it to see where all the gas is going…

 

[Snagglefoot]

Www.fuelly.com shows that it doesn’t get better than 13 mpg.

 

[21_6.7f450]

Those must be hooked to trailers or driven by people that drive like a get away driver after a bank heist in the city. They can get 17mpg... I was only going 70mph so maybe there is that. 85-90mph, all bets are off.
JW, throw sone AMDP trans tuning on your truck. Call Mitch and tell him EXACTLY what you want done. My 10r140 shifted WAY too early. Lugging up hills at 1,100rpms just getting up the hills with 20psi. It was horrible. Night and day different. Should be able to make it shift to your liking.
I wish Ford learned from GM on tuning their HD trucks. Not on the verge of slipping like GM 1/2s but let them rev. Fords don't rev out in normal driving and just run straight to top gear...

 

[Snagglefoot]

That graph was not exclusively highway mileage. It was merely the average of fuel fill ups, divided by the mileage driven between fill ups. Try reset your gas mileage indicator, fill up, and report the average next time you fill up. Almost no one gets 17 mpg as an average unless you do nothing but drive from York to LA each week.

 

[21_6.7f450]

Waco to Houston is close enough. I'd rather not visit NY or LA.

 

[21_6.7f450]

You got me...trucks don't get good mileage...ever...period. it's got to be a little Honda, a stripped out no pulling power 1/2 grocery getter, a Prius....to get over 20.
I was close with this rig over 3100 miles never reset... my F450 averages 13mpg..your Right but a little 4.10 geared F350...sipping that D2 gets close...I spent 3 hours in God awful Houston traffic.

 

[ls1mike]

I have a 24. I use it to pull my 7800lbs travel trailer and the 9000lbs parade float. I have no complaints out west here.
Plenty of power and no weird issue. I had 2 6.0 gassers before and it is just a better truck for towing and comfort.
If you are worried about gas mileage from a gas 3/4 ton an up none of these trucks are for you.
I get about 8 mpg towing and 14ish overall.

It tows much better than my kids LB7 Duramax and by all accounts on the 6.6 board does better than the LLY, LBZ and LML.

You tube has a ton of 7.3 vs 6.6 gas truck videos. Most are pretty decent if you want to get a good idea of what people think.

 

[burbguy82]

That engine/transmission combination would be awesome in a Camaro. No annoying stop/start feature, cylinder deactivation or 0W20. They could even call it a 396 like the original 402 BBC.

would it not be a 400 SB?

 

[64bawagon]

I went from a 2021 with the 6 speed to a 2024 with the 10 speed. Its not an earth shattering difference but it is a better trans. I had no issues with my 6 speed. I did lock out 6th gear while towing my fifth wheel but I also lock out 9 and 10 on the new truck. Mileage is so close its really hard to tell. The 10 spd isnt a magic MPG advantage. I use my truck primarily for travel and towing.

 

[redhat]

Thanks all, I was mostly curious on how the trans performed. I've driven Dads L84 (non-DFM equipped via software, but same DFM lifters) + 10L80 Silverado a ton and have found it to be a decent drive-train. Shifts when expected and doesn't do anything too strange. Was curious how the 6.6/10L100 did.

Unfortunately, Ford is outta the consideration for me. The fiasco behind my PowerBoost and the drive-train woes of my 2019 2.7/10R80 in year 3 of the lease (battery good, truck acted like dead battery for 6 hours un-responsive, trans. shifted from 9th to 3rd on highway, etc.) have ruined my perception. The dealer network is a joke IMO. Stinks 'cause my uncle is always very generous with an A-Plan code.

I agree that you do not buy a 3/4 for fuel economy, but being interested in how it does loaded/unloaded is a fair question. 13-15 MPG if not better is pretty good to me as I am used to a solid 13 MPG presently.

 

[Fahrvergnügen]

I have a 2020 F350 7.3L 10 Speed with 80k miles.

I recently borrowed a 2024 GM 6.6L gas with the 10 speed for a few days.

No comparison the GM’s trans is WAY smoother than the Ford. The GM is like silk while the Ford is like 1 grit sand paper. I’ve heard Allison took care of tuning these transmissions for GM and they were not going to put their name on it unless it was smoooth, and it is.

However, other than the clunky shifts I like the ford better. The GM seems to prefer to hold gears and rev, it actually drove me kinda nuts. The Ford likes to shift early when just cruising around and that 7.3 has great torque down low to lug it around nicely. Reminds me of my early 2000s diesels more than it does a gas truck.

The 6.6 isn’t a slouch but doesn’t have the stump pulling power of the 7.3 IMO. Feels like a big improvement over the 6.0 however, including not looking under the gas tank while filling it to see where all the gas is going…

Understand something here, when a transmission shifts smoothly soft, especially while towing slips the clutches more. You want the transmission to shift as fast as possible and not soft. Second is that that GM trans is Allison branded, not built by Allison. Allison just puts a stamp on it.

 

[21_6.7f450]

If a trans shifted "smooth as butter" Id be very concerned for reasons Fah states above. Even when not towing, in all autos I drive with Tow/Haul, it is engaged. The 6L80 GM drove me nuts,I never could feel it shift. The 10r140, 10L1000, 6r140, A1000, all shift MUCH better when T/H is engaged.
The Dodge 68RE is the odd duck. T/H did nothing to it. It went 320k on the factory transmission pulling 20k all the time, with fluid changes every 25,000 miles with Valvoline ATF +4.

 

[Fahrvergnügen]

If a trans shifted "smooth as butter" Id be very concerned for reasons Fah states above. Even when not towing, in all autos I drive with Tow/Haul, it is engaged. The 6L80 GM drove me nuts,I never could feel it shift.

That’s the reason why it’s not if it’s when you’re going to replace the transmission in a GM. Consumers ruined the automatic as they complained it shifted to hard and now they they wear out clutches faster.