Just did 1,000 miles this weekend, went to snowshoe, west virginia. 75 mph in michigan, 60-65 in ohio, some stop and go, 65 or 70 mph in west virginia, some 55, and back. Mountains ate up some fuel mileage, but for the entire trip I averaged 17 mpg. 18 hours total trip time.
56 mph average, including fuel and food stops. Probably closer to a 65 mph overall driving average.