There is a lot of pissing in the wind here. Bottom line, there are a LOT of variables to safe driving. There is no absolute speed that you are safe if you are under and a lunatic if you are over.
Know the limitations of the driver, of the truck, of your tires, of road conditions and of traffic conditions. Drive only as fast as YOU feel comfortable. For some that is 55, for others like me it is in the upper 60's. Some people feel safe in the 70's and 80's.
Who cares what speed anyone else drives anyways??? Unless you are paying for Turtle n Peeps gas, tires and potential damage to his trailer form a blowout... WHO CARES???
If law enforcement in his area is concerned, they will certainly let him know. Until then, I try not to get my underwear in a twist over something I cannot control.