Do you ever worry about the higher RPMs that it runs in when you pull up long, steady inclines? Mine seems to love to hang in 7th at around 3600 RPMs, which makes me think that I’m hurting the tranny. When towing at 6th gear with that set up you’ve got, may I ask what do your RPMs sit around. Also, do your temps stay at ok levels etc? Much appreciated in advance.
My trailer loves to pull in 6th gear, it will do 7th if I let it but only on flat roads without much wind and then there is no headroom left, almost any dip in the pedal requires a downshift. When I'm on the freeway in 6th gear, I'm at about 2200-ish RPMs. MPH will be in the 63 mph area. I'm too lazy to calculate exact values but that's the general ball park from my memory. The truck will do this all day every day, not a problem.
6th gear in our trucks is "direct" gear, this is the best gear to choose for the life of the transmission (best to avoid overdrive gears while towing larger loads). The vast majority of the time my truck sits in 6th because I use the gear limiter to prevent upshifting beyond that. Hills will drop to 5th, or 4th if needed, but 3600 RPMs up a hill (if that's what you're seeing) is not an issue at all, neither for the engine nor the transmission.
When I'm towing I've learned to slow down. Not just in top speed but also in getting up to speed. Even backing off the pedal lightly can change the demands on the truck so much that instead of spiking to 4300 RPMs it might only need 3500 RPMs. Well worth the time just letting the truck bite instead of forcing it to almost max power.
The difference between the 3.21 and the 3.92, is basically that if I'm towing in 6th, the 3.92 will be towing in 7th at almost identical RPMs. The math works so beautifully for these specific gear ratios, such that they are basically always running the same RPM at the same MPH, except the 3.92 will be one numerical gear ahead. However the final gear ratio used will be almost identical. The exceptions are first gears, and when the 3.21 is in 8th but that will never happen when towing any significant load.
I have no idea how you manage to be at 3600 RPMs in 7th gear. I think you got something wrong there. 3600 in 4th or 3rd makes more sense to me.
My temps are all ok now. I used to see high-ish oil temps but I'm running a 180 thermostat now and that gives me more headroom. My transmission temps always run cool, never seen them get anywhere close to hot.