What kind of MPG's to you see when towing your camper in the Summer Dave ???We have the same transmission cooler, I believe (in my 2012 XL Denali, not my suburban 3500). I usually run in 4th on the highway, pulling 7500 lbs at 65 but my transmission runs 15-20° cooler in mid 90’s ambient temps than what you’re describing, except when pulling up a long grade, but it comes back down to 198-200 pretty quickly after cresting the hill. Similar engine temps though, except mine holds 210° except when climbing. Can’t tell if that’s what you’re experiencing or not.
A couple of things come to mind. One is I’ve read that shifting has a significant impact on transmission temperatures. I read a study on it that showed as much as a 5° increase in fluid temperature for every shift under towing loads. I used to let mine shift between 4th and 5th, but I keep it in 4th now unless I’m on a long gentle descent.
The second thing is related to what John said about oil temperatures and the wise old tuner saying run the engine for cooler oil temps, fuel be damned. In gasoline engines, the amount of heat being sent into the cooling system (and oil) from each combustion event is highest when the AFR is very slightly rich because it results in significantly higher peak internal cylinder pressures. Go richer still and the power output is the same, but the combustion event generates less of a pressure spike, sending less heat into the cooling system. Go on the other side to a leaner burn, and EGTs go up and power output decreases slightly, but it will also result in lower peak cylinder pressures and also generate less heat into the cooling system. I could show you this happening in real time in a piston-powered aircraft with controllable mixture and the effect on cylinder head temperatures is dramatic. It’s related to the speed of flame front propagation relative to piston position near TDC. It could be that enriching the AFR just a little under the typical RPM and throttle settings you would see while towing at 62-65 in 4th, would generate considerably less heat.
Because the radiator and transmission cooling system are linked in our trucks, more heat in the coolant means more heat in the transmission fluid. It’s a wild-ass theory but plausible also. My Derale is good, but significantly less effective than the trucool 40K in my ‘07 XL Denali at keeping trans temps down when towing. That said, its position in front of the radiator send engine temps to the roof when towing (thanks, 6.2), so it’s not a viable solution for us.