58 to 62 when the speed limit is 65 or better. Sweet spot for engine and mpg. Speed limit otherwise or whatever is safe for us and the rig based on road and weather conditions. Retired and am in no hurry, but, I also make sure to let faster folks get around me if I'm on a two lane highway. No point in making the natives angry.
Was in Nevada a couple of years ago and two 5ers blew by me doing at least 80 mph (I was doing 62 and they were going fast enough that I didn't have time to read their license plates as they went by). I passed them 20 minutes later, they were from Nevada...they were both on the side of the road standing there looking at the sidewall damage (substantial) on one of the trailers from a blow out. DW waved as we went by... It just isn't worth it to me. I'm old, but, I'd like to live long enough to be "older".