Food for thought, while there is a lot of discussion of what is legal or not, whether LT or ST tires, what about the reaction and stopping time of traveling at 80 vs 65?
This I believe is a much more important point to make.  Yes you can be a safe driver at 80 but it is always someone else or another variable that will interfere with that.  With no one else on the road in ideal conditions (assuming you are alert, rested, not medicated, no wind, no cars and no blow outs) it can be done.  But.... with any of these variables you are setting yourself up for disaster. 
Plainly said, speed kills.