Probably off*? Montana used to have no limit (1995-1999), Montana and Nevada (1901-1974).

https://www.usatoday.com/story/news/nation/2019/02/22/no-spe...

Honestly on most interstates this seems fine. I don’t mind cars driving fast as long as they stay in their lanes, use signals to change lanes, and merge right if they aren’t passing.

For a lot of Americans the speed limit is only a suggestion, so have it AZ.