Min speed limits are usually 45. These days, if I’m not in a hurry, I go slow (55-60) in the slow lane because CO2 (20-25% lower emissions than the normal 70MPH around here).
However, I also support ticketing for going below the rate of traffic in the fast lane. Examples: 64 in a 65, or even 70 in a 65, but only if there are a bunch of cars behind you, and a big gap in front.
Where? That depends on the jurisdiction. That also may depend on what kind of road we are talking about.