Hey wilco, I was in Motor Transport 1967-1971, and I went to several schools fot MT and eventually I was a Driver Inst. for Busses and TT. I was with the Engs while in V.N. and drove just about anything our Company had. Well anyway, one thing I remember at MT school was that RPMs. are limited for several reasons, one being the Ballancing of the combination of the Flywheel and clutch preasure plate Assembly. Too high of any RPMs. can cause these to Blow apart. Lots of heavy metal comming through the floor causing loss of limbs and then some! Even today in modern trucks rpms are limited for the same reasons, although today things are ballanced a little better. I also have over 2,000,000 miles in Commercial Trucks.
Also balancing of these MVs are not limited to the engine. The drive train is not the greatest when speeds are over 50, or 55mph. Tires were not ballanced years ago on big trucks like they are today.
One more thing, there were no roads in V.N. where we could drive much over 50mph., and Speed does not save lives even in V.N.!! No vehicle faster than a bullit, rocket, or Land mine!!! Good luck, Phil