If I find myself on a freeway or major highway with a 55mph limit, I'll typically drive up to 60 but not over.
On stretches with higher limits, I typically run my rig between 62 and 68 depending on conditions and the amount of ground I have to cover.
But in general, regardless of what I am driving, I typically drive a little slower than the average speed of drivers around me so I don't have to keep changing lanes to pass. I find this less stressful and more enjoyable.
Of course if you asked me 30 years ago, I would have said my speed limit was the car in front of me! :E