I've driven coast-to-coast three times, and from the East coast to Texas many times. In the 1980's we still had the 55mph (88 kph) national speed limit, so driving from San Antonio to El Paso was excruciatingly boring. The only stations on the radio were the million-watt Mexican stations that would overpower any weaker US stations on nearby frequencies.
Not too bad. Most of the east coast is 65 or 70 mph. Out west and middle states you'll find roads marked for 80 mph. No idea about the west coast (California to Washington). For about a year or two after the 55 limit was repealed, Montana had a "reasonable and prudent" speed limit, which attracted people from all over the country to drive recklessly.
And then there's SH-130 east of Austin, which now has America's highest speed limit at 85mph (136 kph). It was built as a toll road, and the Lieutenant Governor of the time was a big Hennessey Performance fan, so he got them to test the toll transponders at 220 mph (352 kph). The toll was successfully read, so don't try and skip out on it by going really fast. :)
I'm sure the Germans here will laugh at our low speed limits, but our roads aren't maintained as well as the Autobahn is (which is amazing, btw), our distances are much longer (Interstate 40 is 4000 km long and it doesn't even go all the way across the US), you can't stay at Autobahn-levels of alertness for a full day of driving, and our drivers aren't as well trained (something I wish we would fix).
From what I heard (even from americans themselves) it's likely because the idiot to good driver ratio is way too high and they'll just start killing themselves.
https://www.youtube.com/watch?v=A4hja0snEHw
https://en.wikipedia.org/wiki/Border_blaster