What does your load weigh? 5000? With 373 gears, let 'er go in OD when you hit 55 and go. Some of the 06 owners run in tow haul most of the time. That's not enough load to cause any special concerns. Your 06 has more/better trans controls than my 04, so this may not apply to you: For the trans,I don't want to pull anything heavier than a lawn mower trailer in OD without the torque converter locked up. For the engine, just listen to your Cummins. It will tell you if its not happy. Go and enjoy and, if you feel like being politically correct, wipe that smirk off your face knowing that you've THE engine/truck for the job. Mark