I'm looking at getting a 2018 Sierra I'm debating between the 3:42 with 8 speed and the 3:73 with 6 speed. I have the 6speed now with 3:42 in my 2016 and I want something better. Just curious how these 2 better options above compare.

I'm honestly not impressed with the 6 speed. Seems herkyjerky.

Math results:



I see no advantage to the 6-speed and 3.73 with all other things being equal. "All other things" includes (and is not limited to):

- Engine
- Altitude
- Induction type
- Fuel-management type
- Tire size
- Cost
- Reputation
- Age of vehicle

The 8-speed starts with a lower overall ratio, even with the taller differential ratio, than the six speed. The first time the 8-speed (+3.42:1) has a ratio similar to a 6-speed (+3.73:1) ratio, the 8-speed has started with greater mechanical advantage and gone through 50% more gears. By the time the 6-speed (+3.73:1) gets to fifth, the 8-speed (+3.42:1) is already partway through sixth. The 8-speed (+3.42:1) has more overdrive choices available along with an overdrive ratio taller than the 6-speed. Reversing will be easier with the 8-speed as well, unless you are looking to set a speed record in reverse.

It'* kind of sad, the 6-speed is a great successor to the 4-speed units, provided it didn't arrive with defects.

In short: more ratios across a wider range that starts lower and ends higher for the same job with the same engine is generally better. In this case the math proves it.

Just spitballing here: maaaaybe if you were choosing a smaller engine with the 8-speed and a larger one with the 6-speed you miiiight see a notable advantage with the 6-speed combination, but that would mostly be because of the engine size more than the transmission+differential combination.

Wow that'* some great feedback! Thank you! I was surfing around last night and it seemed like the 8 speed has been problematic. (More so than the 6) Think that is valid?

Yeah, it had some teething issues. Looking at the dates of the discussions of the problems it looks like the problems started clearing up at the 2017 model year. One thread I saw claims that the dealership actually told them that GM will fix these issues for the 2017 model year, and the 2016 and earlier trucks that have these problems won't get the same fix. The post was made in 2016 so it seems like this is what really happened.

FWIW: The 200-4R and 700-R4 both had teething issues at GM when they were first introduced. The 200-4R got better then they stopped building it. The 700-R4 turned into the 4L60/4L60E/4L65E/4L70E and kept working fairly well for 30 years of production.

I can understand new stuff having teething issues in the real world. The problem at GM is how they handle it. Either way, it looks to me like you can benefit from others' misfortunes since it appears to be better for model year 2017+ .

Awesome thanks again for the opinions! I'm either buying a friends 2017 6.2 or a new 2018 5.3 with the 8 speed!

Hopefully have it Sat! Can't wait!

No prob!

I'm guessing the 6.2 is a 6-speed?

Guessing yes but not sure yet. I've never asked before now. Waiting for him to send me all the details. 2017 premium plus sunroof 6.2 about 20 k miles. Good condition. Has a Waldoc package leveling plus tires. Pretty damn loaded. Black with tonneau cover. 1 minor accident got a new rear bumper.

Thinking it'* worth 35-37k

With lots of factory options I'd guess it is an 8-speed.

Hey bud well I got lucky and found a 6.2 2017 with max tow loaded. Closing deal now. Can't wait to drive out