Speed limits between my house and the DC Beltway vary from 35 mph to 55 mph. People drive about 80 mph. I generally drive 70, and feel like my life is in danger not because I am speeding, but because the flow of traffic is blasting past me like I am standing still.
Even on our narrow twisty 2-lane blind curve every 400' roads, people drive way too fast and pass dangerously. If I can pull over and let them pass, I do, but there are stretches for 5-10 miles where there is zero shoulder, trees up to the road that get their branches sheared off by traffic. No way to pull over there. I've had people pass me and the 4 cars ahead of me, without having a clue what is coming around the curve, over the hill, etc.
I have learned what the posted speed limits are on our local roads. I don't need the GPS for them. But in unfamiliar territory, I generally drive the speed the GPS says, unless the flow of traffic forces me faster. I know the GPS is not 100% accurate, so I am always alert for speed limit signs.
On any interstate, once I am away from a metro area and the traffic thins out, I usually drive between 70-75ish.