In 1975 Jack, Mary Louise, and Susan drove a 1966 Pontiac from Fort Worth to western Colorado, Salt Lake City, Idaho Falls, the Grand Tetons, Yellowstone, Butte Montana, and I think to Glacier National Park, all in two weeks.  This was a year after the highway speed dropped to 55 mph.  As I recall, the Pontiac got 10 or 12 mpg, took old school "Premium" fuel, and the fuel pump went out in the Black Canyon of the Gunnison.  We coasted about two miles downhill and luckily found a service station at the bottom.  Holy moly.