This is actually an excellent question and I think about it often. I'm in Chicago now and the posted speed limit on I90 / I94 is 45 mph. The road is under construction and there are big signs warning of 15 years in prison and $7,500 fines for hitting a worker. What's the prevailing speed? 75 mph. That's right. When there is light traffic, virtually everyone is driving around 75 mph. I noted the speed while my friend was driving me around a few days later.
When I arrived in my MH on a Sunday in light traffic, I drove 45 mph but was getting some poor attitude from the other drivers. I eventually sped up to about 55 to prevent backups but I wouldn't go higher than that.
I have found that it is best to follow the speed limit unless I am familiar with a road or I have great visibility in good weather with light traffic. It's not that I'm overly concerned about getting a ticket. It's just that I've noticed that the speed limit often corresponds to the condition of the road. For example, the speed limit could drop down to 55 just before a curve or road construction.
As others have said, it's safety first, then convenience / courtesy second.