My fire engine and tow truck will both run 65 mph (the sweet spot is about 63 mph) and both have 2-speed rears. Rolling down hill I've hit 70 mph, but that's scary and something I avoid as that's exceeding the limits on EVERYTHING...me and the truck!
If you really cruise above 70 you should concider a modern chassis swap, maybe a Topkick, Isuzu or Freightliner...
Mike B
