[Wombat125]

This is just a guess, but I suspect the reason for the three range preference is that the range selector gearbox is downstream of the hydrostatic motor.

I think folks are saying that in the JD high range, the diesel/hydrostatic system has to work too hard because there isn't enough gear reduction, whereas in the low range the diesel/hydrostatic can't drive the tractor fast enough because there's too much gear reduction.

Someone smarter than I needs to weigh in on this.