Mine started shuttering at 25,000. Mobile One flush did nothing. Torque converter was replaced and transmission is great now--it even got rid of the occasional "clunks" in the parking lot.
My local Mobil has 93 premium with 10% ethanol and 89 ethanol free. I get my best gas mileage from the 89 (22 mpg driving 75-80 with 6.2 NHT). Ethanol free gas has more BTU's (energy) per gallon than 10% ethanol gas. For the same price, I would use 91 ethanol free.
I have 2017 Sierra 6.2/8 speed NHT that developed "the shudders" at 25,000 miles. They changed the transmission fuid but no change. Torque converter was changed 2 weeks ago. Now it is incredibly smooth. There still occasional light clunks between 1-2 gear in parking lots, but I think most 8 speeds do that. I read somewhere that when making the torque converter, the manufacturer was damaging an internal seal and fluid was not flowing properly and that the new torque converters have a different part #. I'm not sure if all of that is true, but I would hate to have to replace torque converters every 25-50,000.
I have a Sierra SLT 1500 6.2 8 speed. I drive 70-80 mph on highway. My truck runs fine on any octane rating though I use premium when towing. Using 93 premium 10% ethanol my high mileage is 21. Using 87 regular 10% ethanol my highway mileage is 20. Using 89 midgrade ethanol free my highway mileage is 22. The 89 ethanol free costs the same as premium, so that is what I usually run since it is more efficient. The cheapest cost per mile is with regular 10% ethanol. Ironically, I had a 2014 5.3 6 speed that only got 19 mpg highway using regular!
An update to my truck. 2017 6.2L NHT Sierra. It started shuddering after 25,000 miles (and a week of towing). First the dealer changed the fluids and I drove it 200 miles. No change in the shudder. Last week they changed the torque converter. It was at the dealer 4 days and they listed 21.5 hours of labor on the warranty claim!! It now is smooth as butter. Still an occasional clunk when cold in a parking lot (another 8 speed issue). Love my truck again.
I got the new transmission fluid and filter yesterday. On the way home there was still a little shutter (especially going uphill at 25-35 mph). This morning I didn't notice any shutter going to work. It is at least 90% better if not totally resolved. I am supposed to drive it for 200 miles--if there is still shutter I will call them back.
A big thanks to this forum for all the information shared. I have a 6.2L NHT with the eight speed. At 25,000 miles I noticed a slight shudder (especially going uphill). Without the info of this forum I would have blown it off. I took it to my local dealer (Everett Chevy in Hickory NC). They made me drive it with Ed, the service manager (who I expected to tell me it was normal). The service manager noticed it more than me and said it was good to address it early. I am scheduled for an inspection/fluid change next week. If that doesn't fix it, then a new torque converter. Thus far a very pleasant experience with the dealer which is why I mention them by name.
I have driven a gas 2500 and a Max Tow 1500 SLT with 6.2. I got the Max Tow 1500. The 1500 has a much better ride (unloaded at least), better fuel economy, and a shorter turning radius--it is a better daily driver. The 1500 pulls my boat (5000-6000lb) as fast as I want (70-80) mph even uphill if needed. I love the large mirrors.
I have the 20" Defenders on a 2014 SLT. Love them. I have the 18# Sumitomo Encouonter HT on a Jeep Grand Cherokee. When i first drove the Jeep I thought the tires were almost identical to the Michelins in ride and quietness. I haven't had them long enough to comment on longevity or traction, but thus far they are a great tire and an even better value.
I have a 2014 5.3 with 6 speed and 3.42 rear end. I also have a 2017 6.2 with 8 speed Max Tow (I believe 3.42 rear end). I can say that my 6.2 with 8 speed and Max Tow does get better gas mileage (about 1-2 mpg or 10% better around town and on highway). Unfortunately, the 6.2 runs best on 93 octane which costs 20-30% more than regular gas. I have mixed 93 and 89 octane with no decrease in mileage. I haven't tried regular gas--I don't want to mess up a good thing.
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