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First Drive: 2015 Silverado, Sierra, & Yukon 8-speed


Zane

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2015_SierraDenali_8L90.jpg

By Zane Merva & Josh Merva
GM-Trucks.com
10/29/2014

 

Last night we had our first opportunity to drive a 2015 GMC Sierra Denali and Yukon Denali equipped with GM’s new Hydra-Matic 8L90 8-speed automatic. In August we drove a 2015 Chevrolet Corvette with the 8-speed but this was our first chance to try the transmission in a K2XX application.

 

As many truck and SUV owners know, the performance of the transmission is a big part in how confident you feel behind the wheel. While only a single part in a complex powertrain, the transmission is the glue that links an engine to the road. When the transmission hesitates, the entire vehicle’s performance suffers. A great vehicle needs an equally great transmission.

 

After only a few minutes behind the wheel it became obvious that shifts are quicker and smoother than the past generation 6-speed. Having two more gears allows the engine to stay in the sweet spot of the RPM range more often. Tip into the throttle and with no drama the 8L90 quickly snaps to the right gear.

 

However, the most noticeable change is how well shifts are managed by the faster next generation transmission controller. Dependable and crisp transmission performance is a benefit in any situation. A slight bump in fuel economy is the icing on the cake, allowing for a +1mpg bump in fuel economy.

 

Under full throttle we found gear changes effortless, quick, and solid. Under light throttle we could barely notice the vehicle had shifted at all. Where the past generation 6-speed sometimes gets confused or doesn't shift as cleanly as we’d like, the new 8-speed was always seemed on point.

 

Unfortunately, we're slightly surprised to see that manual shifting with the 8-speed hasn't changed. Commands to select a specific gear using the shifter stalk are still only a suggestion, with the transmission controller having a final say. In our short test, shift requests sometimes took seconds if the truck didn't agree with the gear we requested.

 

Corvette_8Speed_16.jpg

 

We drove an 8L90 equipped 2015 Sierra Denali and Yukon Denali on urban streets during rush hour. We did not get an opportunity to measure fuel economy.

 

While only available with the 420-horsepower 6.2L EcoTec V8 during model-year 2015, expect this 8-speed to be available in the rest of the full-size truck and SUV line in the near future.

 

When we have an extended amount of time with another 8-speed truck we will report back with fuel economy figures and more driving impressions.

2015_SierraDenali_8L90.jpg

Corvette_8Speed_16.jpg

2015_SierraDenali_8L90.jpg

Corvette_8Speed_16.jpg

2015_SierraDenali_8L90.jpg

Corvette_8Speed_16.jpg

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I absolutly love driving my 2009 Tahoe LTZ with the L9H 6.2L engine, but really get tired of the lazy shifts.

 

Wish I could transplant the new 8 speed or even the new controller in mine..

 

But the good part is I can dust the GOAT & TRD trucks with a soccer mom Tahoe :crackup:

 

TC

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I tell ya, at $50000 a truck, I will stick with the ones I got that are paid for.

 

I love the 6.0 from the 3/4 ton would like to transplant one to my 2000 Z71 with a 5 speed!

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attachicon.gif2015_SierraDenali_8L90.jpg

By Zane Merva & Josh Merva

GM-Trucks.com

10/29/2014

 

Last night we had our first opportunity to drive a 2015 GMC Sierra Denali and Yukon Denali equipped with GM’s new Hydra-Matic 8L90 8-speed automatic.

 

Guessing both with 6.2?

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Funny thing is, I am reasonably happy with my old 4 speed in my 2007 Tahoe... I could only imagine how much better an 8 speed would be. Especially in town, I would not expect a huge improvement in MPG highway...

 

Honestly I wish my 07 would stay longer in V4 mode, any kind of load and it will go right back into V8 mode...

 

John

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So guys, is there a fix for my 6 speed trans? I really don't like the way it shifts with 2000 miles on it I can feel the problems coming??? Thanks in adavance if anyone knows of a GM or after market solution.

Blackbear tune. Supposedly cleans it up real well. I don't have any personal experience but everyone on here loves the performance they get from the tune. I personally have an 02 so I have a 4l60e with a race built trans. by Performabuilt. Quickest shift I've ever felt from an automatic. Chirps tires all the time

 

Sent while in 'Murica...

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Funny thing is, I am reasonably happy with my old 4 speed in my 2007 Tahoe... I could only imagine how much better an 8 speed would be. Especially in town, I would not expect a huge improvement in MPG highway...

 

Honestly I wish my 07 would stay longer in V4 mode, any kind of load and it will go right back into V8 mode...

 

John

 

The 6 and 8 speed transmissions both have a lower(numerical) high gear than the 4 speed does.

 

The 4 speed has a .7:1 ratio in 4th gear, the 6 speed has 0.852:1 ratio in 5th, 0.667:1 ratio in 6th and the 8 speed has 0.85:1 ratio in 7th and 0.65:1 in 8th.

 

The difference may not seem like much, but, with the 4 speed, if you need to accelerate to pass someone, it will likely have to drop down a gear, and that drop is a big jump in ratio to the 1:1 3rd gear. With either the 6 or 8 speed there is another gear (or two) you can use before getting to the 1:1 gear. This should mean better fuel economy when driving highway speeds with other cars around. With the extra gears you also should get better economy in city driving for pretty much the same reason, less throttle required to alter speeds in traffic.

 

Would be interesting to see if GM can get the v4 mode to stay active when not in 8th gear, or when only slight speed increase is needed. Perhaps when in cruise control it will stay longer in v4 mode.

 

My truck came with 3.23 gears and the 4 speed (4.8 engine did not have 6 speed available), and was not a pleasant experience trying to merge into traffic on the highway. It would go from slow acceleration in 4th gear to screaming in 3rd gear by pressing the gas pedal another half inch. You would then lift a bit because it caught you off guard with the downshift, and it would shift back into 4th, and you would then press it the half inch again, this time expecting the downshift, and finish merging. I swapped rear end gears to 4:10 gears, and that made it much more comfortable merging into traffic. I would swap to the 6 speed in an instant if there was a swap package for it.

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