Re: Transmission Fluid Temp.
When towing with any vehicle, it's best to lock out OverDrive and stay in a 1:1 gear or lower. This will aid in cooling all around and engines are built to RUN. If you have the 2V 3.0, it won't have any problems running at 3500rpm on the highway - I know.
Heat is the enemy of any transmission, higher temps over time will break the transmission fluid down quicker. Once the fluid heats up, the internal transmission components heat up even more and the transmission fluid becomes useless and you risk burning things out.
I tow a LOT with my Durango R/T, and with multiple coolers (for almost every fluid), I change my transmission fluid every 30K like clockwork although temps never exceed 200F, even when towing three tons up hill - in 3rd gear (1:1). I don't exceed 68mph when towing.
The mileage deficit isn't very much when towing in a lower gear to keep the engine in the mid-range (or torque band). Again, it will also help lower temps by providing more fluid flow to vital transmission components. Lugging the engine (say, overdrive into a headwind) can cause a lot more harm then using a lower gear to prevent this.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!