I'm going to piggyback on this thread since I experienced something similar today.
Recently acquired, pretty much bone stock 2002 F-250 7.3L supercab with 240K miles pulling around 8K TT. This was a shakedown cruise of a new truck pulling a new camper lasting about an hour out, returning home the same way the following day. Weather was sunny with temps in the mid 80's (F) and mainly traveling on gently rolling. mostly level interstate and beltway roads. At 55 mph, all temps were in 'normal' range (halfway up the gauges and no movement). As traffic sped up, I kept pace and was moving with no issues at 65 which was about as much as I wanted to drive given the slight vagueness of the steering wheel (despite new hub assemblies, drag link and assorted bushings along with a front end alignment but that's for another thread).
I glanced at the trans temp gauge, which I now know is mostly theoretical and saw that the needle had moved up from its usual center point of 100 to 220°. Using the pic from the dieselmann link posted above...
...I watched the needle climb slowly past 230° before topping out just below the upper white line (240ish?). I dropped back to 60 mph with only a minor drop, close to 230° but just above it. Slowing to 55 mph got me to 230° but it dropped dramatically fast to its normal centered position when I hit an off ramp at 50 mph, almost as if the curve had something to do with the temp drop.
Engine temp was rock steady. I got to the campground with no further issues.
The next day, I retraced my route for the trip home. I pushed it to 65 for about two miles and watched the needle start to climb like the day before. I backed off at the 230° point and after that, I kept it at 55 mph and the temps normalized. Then I got home.
After backing the camper into its parking spot, there was a trail of ATF starting just up the street leading back to the parking spot with several large deposits along the way. Maybe two pints, in all.
The dipstick (trans hot and engine running) showed pink fluid all the way to the end of the honeycomb. I let it sit for a couple of hours and then took it out for a ten minute drive with no apparent problems.
My dilemma is that I need to take a longer trip of around 2 1/2 hours into some long uphill pulls (70 West in Maryland then turning left into West Virginia). Not enough time to install a 6.0/Mishimoto/whatever trans cooler and a more reliable temperature monitor as we leave Thursday and I can't get guaranteed shipment in time to install and test them.
I'm thinking that if I stay at 55 mph, I'll pi$$ off some drivers behind me but I can safely make my destination. I'll bring along a few quarts of ATF just in case.
What are my chances of a favorable outcome?