appropriate expressway speed?

Your math is flawed about the 160/day because you can't possibly average 75 mph over an 8 hour day. You would have to drive much much faster than 75 part of the time to make up for slow downs for hills, traffic, rest stops, etc. If my goal is to drive about 58, then for the whole day I might average around 45 or so. If your goal is to drive 70, then you might average around 55. So in my 8 hours I would cover about 8 times 45 = 360 miles. You might cover 8 times 55= 440. So, I guess that would be a significant difference but to me, not worth the extra fuel and danger.