One reason I have been told by a mechanic for the 65 MPH limit is both a bit about protecting the drive train but also to avoid foaming of the tranny fluid. He said occasional excursions above 65 aren't a big deal such as in the case of short duration speed increase on a down hill.
Take that for what it is worth.
Don, Sandee & GSD Zeus. Guardian GSDs Gunny (7/11/15) & Thor (5/5/15)
2006 2015 DSDP 4320 4369, FL Chassis, 2013 CR-V 2020 Jeep Overland, Blue Ox Avail, SMI AF1.