GMC Sierra 6 or 8 speed transmission

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mossyoak

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I’m looking at a 2018 GMC Sierra and they have 6 or 8 speed transmissions. Anybody have one or experience about which is best for fuel, towing etc...


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S

Smo

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Re: GMC Sierra 6 or 8 speed transmission

I own an 06 model with the Allison transmission I think it’s a 6 speed.

I’ve never had any problems with it and bought it new.

I get 16 mpg overall, keep it at 55 mpg and got up to 21 mpg.

If you can afford a new one, you can afford to feed it! LOL

Great trucks all and all. Best ride of the 3 major brands.

I drove them all before buying mine.
 
M

mossyoak

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Re: GMC Sierra 6 or 8 speed transmission

Smo":1vgjtkeq said:
I own an 06 model with the Allison transmission I think it’s a 6 speed.

I’ve never had any problems with it and bought it new.

I get 16 mpg overall, keep it at 55 mpg and got up to 21 mpg.

If you can afford a new one, you can afford to feed it! LOL

Great trucks all and all. Best ride of the 3 major brands.

I drove them all before buying mine.



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griz01

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Re: GMC Sierra 6 or 8 speed transmission

I believe all the 2500HD are 6 speeds. Only way to get the 8 speed is in the 1500. If using for any heavy or long distance towing, probably no the best choice. 2500's do get worst mileage but "best" towing and mileage just don't seem to be workable. Lowest numerical value axles with lowest numerical overdrive will get the best mileage but won't tow worth anything. Highest numerical value axle gives great towing but poor mileage. Every thing in between is a compromise. What you do with the truck is important. If you run max load all the time, determine how heavy the max load you will be running and try to get an axle setup with the highest numerical value and the biggest engine you can and forget the mileage. If you will running. like most truck owners, empty or with very light loads and only tow a couple of times per year, pick axles that put your max load at the upper limit of the towing range and a smaller engine to get the best mileage your can. Most trucks have a switch to lock out the overdrive when towing anyway so 6 or 8 well that depends again on what you want to tow. Heavy loads consistently, probably won't see much difference. Light loads and limited towing, definitely the 8 speed.
 
M

mossyoak

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Re: GMC Sierra 6 or 8 speed transmission

griz01":3e1gkkbx said:
I believe all the 2500HD are 6 speeds. Only way to get the 8 speed is in the 1500. If using for any heavy or long distance towing, probably no the best choice. 2500's do get worst mileage but "best" towing and mileage just don't seem to be workable. Lowest numerical value axles with lowest numerical overdrive will get the best mileage but won't tow worth anything. Highest numerical value axle gives great towing but poor mileage. Every thing in between is a compromise. What you do with the truck is important. If you run max load all the time, determine how heavy the max load you will be running and try to get an axle setup with the highest numerical value and the biggest engine you can and forget the mileage. If you will running. like most truck owners, empty or with very light loads and only tow a couple of times per year, pick axles that put your max load at the upper limit of the towing range and a smaller engine to get the best mileage your can. Most trucks have a switch to lock out the overdrive when towing anyway so 6 or 8 well that depends again on what you want to tow. Heavy loads consistently, probably won't see much difference. Light loads and limited towing, definitely the 8 speed.



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rem270

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Re: GMC Sierra 6 or 8 speed transmission

I'd be more worried about that flex fuel 5.3 than anything. They all use oil. My 2011 did, my buddies 2011 did. Another guy I know bought a new 2016 and it was over a quart low before the first oil change. I'd recommend go having the flex fuel turned off as soon as you bought it. My buddy talked to a GM mechanic about it and he said if someone buys one to always check the oil or have it turned off because they all do it. Lots of good stuff on google to read about also with folks with the same problems.
 
double browtine

double browtine

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Re: GMC Sierra 6 or 8 speed transmission

I have heard the rebuild cost of the 8 speed transmission is alot more than the 4 and 6 speed.

On the oil consumption that rem270 talked about, I'm a Chevy guy and all Chevy engines have burned oil through time. But an old engine builder once told me that if it doesn't burn oil, it isn't gonna run good. He was an old school drag racer.

I check my oil every other time I fuel up. Also, all of these newer vehicles use 5w-20, 5w-30 oils. They are alot thinner, therefore the engines burn more oil. Engine and bearing tolerances in today's newer engines are designed for using it. When towing, engines work harder and more oil is used.
 
jlanecr500

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Re: GMC Sierra 6 or 8 speed transmission

I've had 2 of the flex fuel 5.3's and had no issue with oil consumption. The 2014 engine saw refinements carryed over from the performance engines like aluminum block, variable valve timing and redesigned 4 cylinder mode. I think the key to the success of the 2 I owned was the fact that I changed the oil at 50% oil life by the DIC. And I didn't cut it any slack after the first 500 miles. These engines come with synthetic oil and as we all know, the rings do not seat well with synthetic. Most mechanics advise dino oil until after break in. I just run the pizz out of it.
Anyway, back to the tranny. I towed nearly 10k pounds with the 5.3l 6 speed without any trouble. I don't think I would do that with the 8 speed just yet until they get the bugs worked out.
I build transmissions for myself and friends, and can say that it is all about design/engineering When it comes to longevity. I would go with the 5.3 and 6 speed.
 
griz01

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Re: GMC Sierra 6 or 8 speed transmission

As you probably guess form my previous post, I just went through this myself. Did tons on research on all HD truck models. I just got rid of my 5.3 because of piston slap. never had any problem with oil use though. I just couldn't take the racket any longer. They tell me that is common with the Vortec motors due to the assembly specs they use. GM told me that piston slap is "normal" and NOT covered by any warranty! My new truck is NOT a GM product for this exact reason.
 
cecil30-30

cecil30-30

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Re: GMC Sierra 6 or 8 speed transmission

Just get a ford and don't worry about it burning oil. I think it's crazy for a engine to burn oil, let alone a brand new engine burning oil. My 02 ford explorer has 213k on it, I change the oil every 3-5k and it's always full when it's due to be changed again. I also have a 97 f250 7.3 powerstroke with 190k, and it doesn't burn oil either. My wife drives a 06 trailblazer with the inline 6, it has 150 on it, and it doesn't burn oil. It's my first chevy, so far it's been a decent vehicle. I know this post was about transmissions but I couldn't hold my tounge on a new engine burning oil and it being normal. Carry on

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hillbillyfab

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AFM-Active Fuel Management are the ones I've been around that had the heavy oil consumptions. Not sure if that's the same as the flex fuel versions.
 
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mossyoak

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Re: GMC Sierra 6 or 8 speed transmission

Thanks for yalls response


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Robo Duck

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Re: GMC Sierra 6 or 8 speed transmission

Buy the 6.2 with 8speed . You will love the power if you tow anything . 21mpg .
 

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