The speed limit is the top speed that is legally allowed. If it is exceeded, you are breaking the law.
The speed limit of 80 mph is the fastest that vehicles are allowed to go. So 60 mph would be normal for trucks.
about the same speed as freeway traffic
It would be 65 MPH.
"The maximum speed limit on the freeway is 70mph."
There isn't a minimum speed per se, but if you are impeding traffic you can be ticketed. It isn't safe to drive slow on the freeway, other cars may not judge your speed correctly, it's more difficult to merge, and it makes other drivers very upset, which makes them more dangerous and aggressive. So if you can't drive the speed limit on the freeway, stay on surface streets or take the bus.
The highest speed limit I know of on an Interstate highway in the U.S. is 75 MPH, but that varies.
70 mph. But the turnpikes are 75 mph.
If it's not the speed limit yes
If the speed limit is 50, drive 50. If the speed limit is 60, drive 60 and so on.