6.6 6spd towing..some real world data.

TeamSaris · November 30, 2023

First off, I love the 6.6 gas. We tow our race boat (9000lb boat, 2000lb trailer) all over the country with it. Im sure a diesel would tow it better, but ive never felt like it needed a ton more power on a hill or passing a truck. We used to haul it with a 6.0 gas. I cannot describe to you what a major step up this was lol.

Recently I hauled from upstate NY to south FLA and back. Pretty much 3000 miles. Im a fairly agressive driver. Cruise was never set below 70 and in SC, George and FLA was set at 77. Through a few traffic jams, weather changes and all of it we average 9.8mpg. Pretty good all things considered.

I have a 10spd 6.6 on order for myself. Once its in ill provide a back to back. That being said, the 6spd is a really great trans for this truck.

Pryme · November 30, 2023

If your current truck isn’t super high mileage, why replace it? Maybe keep it around in case the new one is not all you hoped it would be, and sell it?

TeamSaris · December 1, 2023

It isnt mine. Its the race teams. it will remain the main tow rig. I ordered a 2500 for myself

Pryme · December 1, 2023

21 minutes ago, TeamSaris said:

It isnt mine. Its the race teams. it will remain the main tow rig. I ordered a 2500 for myself

How fast is the race boat flat out? What’s powering it?

TeamSaris · December 5, 2023

around 100. 700hp 572cid big blocks.

www.teamsaris.com is our company

Pryme · December 6, 2023

20 hours ago, TeamSaris said:

around 100. 700hp 572cid big blocks.

www.teamsaris.com is our company

That would be so fun. I’ll check it out

TT331FB · January 12

2021 6.6 Gasser towing 10,000# travel trailer. This trip, 2000 miles, getting average 9.6mpg. No oil use, 40% life yet to go and I haven’t added any oil sence the change. I’m very happy with this combination. We are traveling from Idaho to Florida. Tranny running 195 degF every time I look.

Another JR · January 12

Might want to get the newer 153F transmission thermostat at some point since you tow heavy. I’m putting it in later this year when I do a fluid change at 15k miles on my 2021. I don’t tow a heavy trailer so I haven’t been in a hurry.

Cold North Country · January 12

Is the GM oem 153 Thermo readily available, and if so where?. Dealer said will not replace under bulletin unless there is a problem. Why does GM want the Tranny to run so hot? Is there a reason?

TT331FB · January 12

I had a 2003 GMC 8.1l w/allison 1000 that ran 210 degF. When I pull hills on the 2021, I have not seen higher than 200. I’m not concerned but I will look into your suggestion.

Another JR · January 13

3 hours ago, Cold North Country said:

Is the GM oem 153 Thermo readily available, and if so where?. Dealer said will not replace under bulletin unless there is a problem. Why does GM want the Tranny to run so hot? Is there a reason?

The new part superseded the 195F thermostat in mid 2021. All that are out there now should be the new ones. They have a code stamped on them. I think it is “70” but I’d have to look again. The new part should be readily available. There was a long thread on this.

I suspect the original reason the engineers designed it for 195F was for slightly better fuel efficiency. I’m guessing when they saw the degree to which the system temperature climbs above the set point in a long pulling climb in warm conditions, they decided they should open the cooler flow up at a lower temperature so you start hot climbs 40F cooler. Since most customers want longevity over 0.01 more mpg, and lower transmission fluid temperatures probably improve longevity, many have changed the thermostat, some with the dealer being willing to interpret the TSB to cover it even with no transmission “malfunction.” My dealer refused. It’s easy to do yourself. One person in the prior thread said almost no fluid is lost when changing the thermostat. I’m going to change mine right before my first fluid change so the fluid level will correct.

TT331FB · January 13

This is what doesn’t make sense…the tranny is cooled with engine jacket water. Cooling water comes off the engine is at 210. What comes off the radiator is about 180. I’m not sure how you can get the tranny any cooler than the average of (210+180)/2 or 195 which is what mine runs. A lower set Tstat would be open all the time but would not help cooling at all.

Homewrecker007 · January 20

On 1/12/2024 at 9:01 PM, TT331FB said:

This is what doesn’t make sense…the tranny is cooled with engine jacket water. Cooling water comes off the engine is at 210. What comes off the radiator is about 180. I’m not sure how you can get the tranny any cooler than the average of (210+180)/2 or 195 which is what mine runs. A lower set Tstat would be open all the time but would not help cooling at all.

That is not the case. I have a truck with the lower temp thermostat in it and it stays around 154-160°, my other truck has bypass kit in it and it runs 100-145° depending on weather and driving style. The transmission thermostat is only closed till the trans temp gets to set point and opens, then it's open the whole time.

TT331FB · January 20

There must be an auxiliary tranny cooler then.

Homewrecker007 · January 22

On 1/20/2024 at 5:06 PM, TT331FB said:

There must be an auxiliary tranny cooler then.

If you have one it will be right out front most generally in the center of the A/C condenser. Also the dash water temp gauge is inaccurate. 210 is probably closer to 195 give or take.

Edited January 22 by Homewrecker007