I've been through ID and Utah where speeds were 80 mph for autos. Even when I had my 2500 diesel and light 9200 lb 5th wheel with 87 mph tires I still only went 65 mph. I just couldn't see running 75 mph and sucking diesel like a snow cone on a hot day.
Last year heading south from ID into Utah with my gasser and 9500 lb TT I still kept the speed under 65 mph. I never felt like a hindrance to anyone. It's not like traffic in L.A when out on the interstate in ID and UT. I did suck gas big time last year but it was really cheap. :)