This will help figure speeds.
http://www.drivetraindirect.com/t_gear_calculator.htm I don't know for sure what speeds a 5.9 likes to be run, but your numbers sound high to me. While I do believe it will turn that fast, you will burn lots of fuel, and shorten the life.
For best fuel economy you want the rpm at your preferred speed, say 68 mph, to be the same as, or 100 to 200 rpm over the speed that the engine makes it's peak torque. I am thinking 1800 is about right for the 5.9, but do some checking to be sure.
I don't know of any 19.5" mud tires. The most aggressive I could find about 3 years ago when I bought a set for my truck were Good Year G171 in 8R19.5. they are pretty tall, about 32-33" and pretty skinny, I liked them and they suite me fine, but I would not call them mud tires.
I just recently heard that the G171 tread design is no longer being manufactured. The replacement (not new, but the closest thing still available) is a much less aggressive G124. Perhaps there are some new better tread designs out in 19.5, or there should be soon, as I think the are starting to use that size rims on new Ford regular pickups?
Grigg
