Very outdated information from the 70's compiled from biased testing. In the 70's a car doing 60 mph was turning around 3500 rpm's or higher. Today my SUV will do 75 mph without going over 2100 rpm's.
During the summer I drive to work from the lake quite often. Traffic in the fast lane moves consistently at 75 to 80 mph and see almost no unsafe driving incidents. About once a week someone will hang in the fast lane doing 68 mph and all hell breaks loose. You see more unsafe driving incidents in 10 minutes than you see in a week at the higher speeds.
Recent studies show that the safest traffic pattern on non-urban highways is when the lanes move at the speed which 80% or more of the drivers want to travel at. Traveling at other rates of speed increase tension, increase reaction time and promote compulsive actions.
I'm NOT complaining about safety at all. Would just like to see a new study of the fleet to find the average most efficient speed and go with that.