On our last few jaunts south, Montana interstates have big chunks where 90-95 mph seems to be the de facto cruising speed. Idaho seems to be 85-90, except for the Idaho Falls-Pocatello stretch of I-15, and over closer to Boise on I-84. Ditto for Utah north of SLC, but it bumps up to 90+ south of Nephi. US 93 and Nevada 318 are good for 85-90 cruising. When you can pull 18 mpg (US) with the cruise set at 85 mph, you can put a lot of miles in the mirror in a day, with a 25 gal tank.