IIRC, the Allison 5 speed of certain varieties has an aftermarket conversion to make it 6 speed double over. Might check with John Kennedy on that. Be patient, some one will comew along with your answers. In the mean time, gear ratio and tire size might help. I remember a link arround here to a conversion table to determine rpm at a a given speed.

Good luck and happy motoring.

A quick search and I found 1 Gear/RPM Calculator

Hope this helps. If not there was another one on 4x4 site.