It really depends on what state you live in as each state has general levels of acceptable speed.
This.
Behaviors I've seen:
Southern Louisiana: If you're in the right lane, you're 5-10mph below the limit.
If you're in the left lane, you're driving at the limit.
Otherwise, you're off the road, getting a ticket. Srs business.
Pennsylvania: 5-20mph over the limit is normal, depending on where you are.
On the route I take to work, traffic in the 25mph sections usually move at 35mph. 35mph limit -> 45mph. 40 limit -> 50mph.
55 highway -> 65-70.
65 highway -> 75.
School zones: 15mph -> 15mph. They mean business there, and there really are kids walking in the area. (Contrasted to the school zones at my highschool and middle schools. The street was about 200ft from the school, and no one was ever out on the lawn there anyway. No one walked either. It was buses or be driven by parents. Or walk 2hrs.)
San Francisco, CA: ~65mph limit -> 10mph actual speed. Or get out and
walk. Christ.