I'm not sure if I should post this here or in the Ford section.

The other day I was towing a half ton GMC on a U-Haul trailer with my F250. The truck has 3.55s and a ZF 5 speed. Anyway I figure that the combo I was towing had to weigh at least 7000 lbs. On a fairly level section I was able to tow it at highway speeds in overdrive, the engine did not struggle at all and no more throttle was required than when empty.

My question is this: Is fifth gear in the ZF transmission strong enough for sustained towing of heavy loads or is this something I should avoid?

I would think if you were using no more than normal throtle then you should be OK. What does the owners manual say about towing with the 5 spd trans?:D

Yeah, you're fine.. Don't confuse the auto tranns O/D with the manual tranny.. Their problem is heat created by the torque converter unlocking, but the tranny staying in O/D.

I haven't heard of anyone fawking up their tranny by towing in 5th. Unless you're an oil-burner, you'd probably have to go down to 4th when you hit a hill anyway!