I ran a small fleet of F250HD's towing ridiculous! loads on gooseneck trailers from the late 70's to the early 90's. The sweet spot for 460's mileage seemed to be 3.08's direct running solo but most of ours were 4.10's. To many RPM's is overspeed, too few is lugging. Not sure what the OD ratio is on a ZF is but you can go from there.