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Message: [QUOTE="Charles, post: 202593, member: 103"] The load that I roasted the first ZF6 with in my Red truck was usually my F-350 trail truck on a bumper pull equipment trailer. It was only ~20,000 gross, but I rocked 75 to 80mph back in the day, not realizing the driveline was dying. It turned the fluid dark black in the trans and the rear differential and pushed the needles in the u-joint caps into the trunions....twice.... I gave up, replaced the trans with a 4R100 and rebuild the rear diff. I have since replaced a driveshaft u-joint from heat again, but the trans has never gotten hot like the ZF6 did. In the 550 the hardest pull is the usual load of 18 4 x 5 round bales on a 24' gooseneck at ~75mph. About a month ago I swapped out the ZF6 because it wouldn't stay in gear anymore and the fluid had scorched bad and hadn't long since been changed. About a week ago I rebuild the third member in the Dana 135 because it melted the ring side carrier bearing until the ring gear was no longer in contact with the pinion. The moral of the story is that a well tuned 7.3 will see the demise of the driveline in one of these trucks, and that the 4R100 seems to be able to hang in there longer than anything else when the continuous load of full power towing is allowed to run its course for a few hours at a time. The ZF6 and the axles give it up. The 4R stays the course. I have towed the identical trailer of hay with my Red truck at a higher speed and while I completely emptied the water injection tank [b]3 times[/b], the trans fluid still looks brand new. At this point I'm thinking of getting a little single axle tractor to replace the 550. It is apparently not enough truck to support its own engine. [/QUOTE]