appropriate expressway speed?

One of the posts claims that you really can cover lots more miles per day (160?) going faster. I don't buy it. It's not your fastest speed that depends on how far you can go but your average speed for the day and the difference in the average between 70 or more and 58 or 60 is only going to be a few miles/hr. maybe 25 or so miles more in a day, not 160. Stops and slow downs for hills and traffic will bring the averages closer together. Unless, you drive non-stop for the entire day and don't slow down for anything and that's no fun and dangerous. We've all had RV's pass us only to see them again at a rest stop down the road.