SOme states like Washington, have two interstate limits in "SOME" areas. IE 70 in lower population/fewer exit/on ramp areas, FOR rigs UNDER 10K gvw, NOT towing. If over 10K gvw, and/or towing, then max 60mph or in some area's, like downtown Seattle, 55, parts of Tacoma in a construction zone 50mph.....
I find 60-62 is nice on cruise. As noted, 65+, clobbers mpg's, yeah we are not supposed to worry about them.....reality, some of us do like to get the best mpg, for a given speed area driving etc.
If you have a dually, or ANY SW pickup with a license over 10K gvw, your limit towing or not is 60 mph!
Marty