6 Speed vs 8 Speed

jancebc · 2026-07-15T01:22:12Z

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

CamGTP · 2026-07-15T02:48:55Z

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.

davester · 2026-07-15T08:56:48Z

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.

newdude · 2026-07-15T12:47:38Z

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 4 hours ago by newdude

Grumpy Bear · 2026-07-15T13:51:08Z

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 3 hours ago by Grumpy Bear

newdude · 2026-07-15T16:23:34Z

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 1 hour ago by newdude