Towed for the first time with my truck over the weekend. Towed approx 6k. Double axle trailer with tractor, bush hog, and disc. Towed it about 100 miles over various terrain and I can't be more pleased with the performance of the truck. I was anxious to see how the 5.0 handled it and it did great. Never once seemed underpowered or uncontrolled. There were times cruising down the interstate I would have to slow down because i didnt realize how fast i was going. There was one hill that is about a 3-4 degree grade and about 3/4 of a mile long. With the tow haul mode on it downshifted to 3rd, got it in the power band and pulled 65mph to the top and had i gave more skinny pedal it would have picked up speed. I know it wasn't a huge load and its rated to tow a good bit more but i was excited to see it handle it with such ease. Also, the 6 speed, tow/haul mode and the select shift is an excellent combination.

 

With your 3.73 axle ratio, that's about what I would expect. 6k is a good load, and you were probably at the end of your payload capacity, but you still had some pulling capacity remaining.