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6 Speed vs 8 Speed

jancebc
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jancebc New Member

jancebc

Posted
Posted

I’m in the market for a K2XX 6.2 truck and I’ve heard some conflicting things about reliability. My mechanic says that the 8 speed is just not consistent at all in reliability and that some people can be ok and others can’t. Given this I was hunting for a 2014 only truck with the 6.2 (which is quite hard to find) and I’m coming on here to ask if the 8 speed is really that bad. I need to expand my years so I can find some nice trucks but I don’t know if it’s better to wait on a 2014 to pop up or to expand my field and get some more deals.

 

Thanks in advance for any feedback and comments 

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CamGTP Senior Enthusiast

CamGTP

Posted
Posted

The 8 speed isn't a bad transmission at all. They just have common issues like any transmission.

 

Ideally you'd like to find a low mileage truck or find a truck with good service history or maybe even a factory rebuilt transmission already in it.

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davester Senior Enthusiast

davester

Posted
Posted

I would say the main negative would be that I believe it's around when the 8-speed was introduced, so most likely it will have the original parts, whereas newer trucks will have 8 speeds with various upgraded/improved parts that fixed various issues with it.

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newdude Senior Enthusiast

newdude

Posted
Posted (edited)

8 > 6

 

Your mechanic needs to have hung around a dealer or transmission shop over the last 5-10 years.  He'd change his mind.

 

6L80 has a higher issue rate for sure.  Torque converters, fluid pump gets wiped from converter failure, 4-5-6 clutches burn up, the TEHCM gets wrecked from the clutch debris from converter failure, some burn up hard parts too, etc.   

 

8L90 has its quirks but if you change the fluid regularly and use the right fluid, its been a more reliable unit over the 6L80.  

 

 

Edited by newdude
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Grumpy Bear Senior Enthusiast

Grumpy Bear

Posted
Posted (edited)
1 hour ago, newdude said:

8 > 6

 

Your mechanic needs to have hung around a dealer or transmission shop over the last 5-10 years.  He'd change his mind.

 

6L80 has a higher issue rate for sure.  Torque converters, fluid pump gets wiped from converter failure, 4-5-6 clutches burn up, the TEHCM gets wrecked from the clutch debris from converter failure, some burn up hard parts too, etc.   

 

8L90 has its quirks but if you change the fluid regularly and use the right fluid, its been a more reliable unit over the 6L80.  

 

 

 

Curious @newdude, how does this change when a 6L80 is either fitted with a billet converter OR is behind a V6 whose converter isn't a pile of junk? Oh, and are you saying in the above the 4-5-6 clutch's burning up is the result of the converter/pump induced failure or they are just fragile? Thanks in advance for any feedback you may be willing to give. 

Edited by Grumpy Bear
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newdude Senior Enthusiast

newdude

Posted
Posted (edited)
2 hours ago, Grumpy Bear said:

 

Curious @newdude, how does this change when a 6L80 is either fitted with a billet converter OR is behind a V6 whose converter isn't a pile of junk? Oh, and are you saying in the above the 4-5-6 clutch's burning up is the result of the converter/pump induced failure or they are just fragile? Thanks in advance for any feedback you may be willing to give. 

 

 

4-5-6 fails due to design.  Not enough clutches and not enough piston to clutch surface area.  Also the backing plate can bow.  

 

I'd say converter is two part.  The design of converter chosen, and factory tuning of it aka slipping it in D (also L6) for AFM purposes, as well as locking it up in 2nd gear, sometimes 1st, also it will partially lock up at light loads.  All because CAFE/fuel economy requirements.  A billet converter on the stock tune might hold up better, but would still be subject to the stock tuning and how its designed to lock or partially lock.  

 

The most baffling part to me, is the 2007-2013 6L80s didn't have the failure rate the 2014-2018 ones do.  I've been where I work for almost 17 years now, and I recall maybe 3 or 4 of the 07-13 transmissions getting pulled for something.  14-18 on the other hand, it was at a minimum one a month being opened up. 

 

If I had to guess, I'd say the tuning for the gen 5 engines/6L80 setup and its fuel economy goals had a good bit to do with it.  Also, it was bankruptcy time during these years, so I'm thinking some cost cutting was done for the launch of the K2XX trucks.  See also now the 10 speed valve body issues (more cost cutting).  They blow out some mesh screen in the spacer plate, undersized check ball, and the feed limit valve was aluminum and wears out prematurely.      

 

I don't think it was the HP jump from the gen 4 to gen 5?  The 6.2 in the 07-13 trucks was good for 380-400hp during its life/states of tune, and the 5.3 did 310-320hp.   

Edited by newdude
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TheRiver Enthusiast

TheRiver

Posted
Posted

IMNTBHO;  the worst thing for the 6L80 is to be lugged down to low engine rpm while in high gear......AKA, to save fuel.   I rarely use D on my '16 with the 6L80 and use M mode to keep engine RPM above 1500 at the speed I'm driving, which is also benefiticial for the tranny because line pressure is kept up because the pump is actually turning a decent speed, and it's not always hunting to drop to the next gear to lower the engine RPM's.

 

Never above 30mph.............M3, maybe M4 if just driving thru a small NoDak town

Never above 40mph............M4

Never above 50-55mph.............M5

 

This is way better for the tranny and the engine.........especially if AFM is still active.

 

262K miles on my OEM 6L80 as I type this.

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newdude Senior Enthusiast

newdude

Posted
Posted
2 hours ago, TheRiver said:

IMNTBHO;  the worst thing for the 6L80 is to be lugged down to low engine rpm while in high gear......AKA, to save fuel.   I rarely use D on my '16 with the 6L80 and use M mode to keep engine RPM above 1500 at the speed I'm driving, which is also benefiticial for the tranny because line pressure is kept up because the pump is actually turning a decent speed, and it's not always hunting to drop to the next gear to lower the engine RPM's.

 

Never above 30mph.............M3, maybe M4 if just driving thru a small NoDak town

Never above 40mph............M4

Never above 50-55mph.............M5

 

This is way better for the tranny and the engine.........especially if AFM is still active.

 

262K miles on my OEM 6L80 as I type this.

 

 

All valid.

 

But.  How many people really do this?  Its few and far between.  Most just drop it into D and go.  

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Grumpy Bear Senior Enthusiast

Grumpy Bear

Posted
Posted
19 hours ago, TheRiver said:

IMNTBHO;  the worst thing for the 6L80 is to be lugged down to low engine rpm while in high gear......AKA, to save fuel.   I rarely use D on my '16 with the 6L80 and use M mode to keep engine RPM above 1500 at the speed I'm driving, which is also benefiticial for the tranny because line pressure is kept up because the pump is actually turning a decent speed, and it's not always hunting to drop to the next gear to lower the engine RPM's.

 

Never above 30mph.............M3, maybe M4 if just driving thru a small NoDak town

Never above 40mph............M4

Never above 50-55mph.............M5

 

This is way better for the tranny and the engine.........especially if AFM is still active.

 

262K miles on my OEM 6L80 as I type this.

 

I started doing this late last year as a habit and situationally for much much longer. Coincidently I've found 1500 rpm to be my trigger point as well. 

 

It's the same sensation as a manual held in high gear at low speeds. You can feel it tearing itself apart. Don't let it! 

 

I'll add this tidbit. When on a longer run at 50-55 mph and in M5 once up to speed I tap the tow button which will hold it in 5th gear and then ask for M6 to activate the AFM. When I have to slow to a speed that will downshift I go back to M5 then tap out of tow. Smooth as butter. I still use D for Interstate work above 54 mph. If I forget, it will remind me. LOL. 

 

 

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Grumpy Bear Senior Enthusiast

Grumpy Bear

Posted
Posted
19 hours ago, TheRiver said:

262K miles on my OEM 6L80 as I type this

 

Nice! And thanks for the assurance I'm helping Pepper and not hurting her. 😉 

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swathdiver Senior Enthusiast

swathdiver

Posted
Posted
22 hours ago, newdude said:

The most baffling part to me, is the 2007-2013 6L80s didn't have the failure rate the 2014-2018 ones do. 

 

Heat is what kills them, the redesigned cooling system and that full temperature thermostat on the side of the transmission doomed them from day 1.  The heat eventually causes the clutch material on the plate to flex which then breaks up and runs throughout the transmission.  

 

The GMT900s when serviced regularly, often run into the 200K mile range or more.  One 6L90 in an HD3500, pulled into the shop on its own power for overhaul at 320K miles at Precision Transmission a few years ago.

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newdude Senior Enthusiast

newdude

Posted
Posted
13 minutes ago, swathdiver said:

 

Heat is what kills them, the redesigned cooling system and that full temperature thermostat on the side of the transmission doomed them from day 1.  The heat eventually causes the clutch material on the plate to flex which then breaks up and runs throughout the transmission.  

 

The GMT900s when serviced regularly, often run into the 200K mile range or more.  One 6L90 in an HD3500, pulled into the shop on its own power for overhaul at 320K miles at Precision Transmission a few years ago.

 

 

Yes I did forget that part.  

 

That said, the older ones would still get in the 170-190F range.  Not that far off from an early K2XX.  

TheRiver Enthusiast

TheRiver

Posted
Posted

And to top it off finally.......................these small GM LS engines are designed to SPIN!!!!!  Why choke them down to just off idle for cruising.  Why/how the "bean counters" have influenced the "engineering department" at GM is beyond me.  

They took a solid platform of an engine and turned into a joke in OEM form!!!

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swathdiver Senior Enthusiast

swathdiver

Posted
Posted
On 7/16/2026 at 10:43 AM, newdude said:

That said, the older ones would still get in the 170-190F range.  Not that far off from an early K2XX.  

 

At peak temperature.  They get to cool down and warm up more, especially those with the external cooler.  I have three GMT900s and one of them has over 225K on its original transmission.  Fluid gets changed about every 36K miles, all of it.

 

 

On 7/17/2026 at 6:26 PM, TheRiver said:

And to top it off finally.......................these small GM LS engines are designed to SPIN!!!!!  Why choke them down to just off idle for cruising.  Why/how the "bean counters" have influenced the "engineering department" at GM is beyond me.  

They took a solid platform of an engine and turned into a joke in OEM form!!!

It's not the bean counters, it was the government.  They had to meet CAFE standards and other government requirements.

 

To get around that, we tune them and modify them to our liking.

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truckguy82 Senior Enthusiast

truckguy82

Posted
Posted (edited)
On 7/16/2026 at 10:43 AM, newdude said:

 

 

Yes I did forget that part.  

 

That said, the older ones would still get in the 170-190F range.  Not that far off from an early K2XX.  

10’s of thousands of k2xx 6l80’s probably had bad thermostats as well. I had one.

 

I would have never known my thermostat was non functioning had I not been a forum member and known that it’s supposed to open at 190 and level off. Also technicians would never know either because how often do they drive them enough to reach 190 and even notice when they get there.

Edited by truckguy82
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