[slow]

I was not able to find a torque curve for your engine, but most gassers' torque curves would suggest that you will be north of 3,000 rpm in heavy towing conditions for best performance. Anything less in rpm and the curve is too steep resulting in the tranny seeking another gear. With our 6.2 F250 in the mountains, I find that 3,800 to 4,200 rpm is a sweet spot where speed is maintained with no shift searching.