I am pretty much always racing my own times everywhere I go. My car has a tripmeter that shows average speed, time, fuel economy for the trip, etc. I can average 60mph easy, stopping for pee breaks every 30 minutes. 65, not hard, 70, doable, 75 is getting difficult. 80 requires a lot of dedication. 85 is almost impossible over 50 miles. It just gets exponentially harder from there. Just to offset 30 minutes of fuel stops, you would have to AVERAGE 150mph for nearly 1.5hrs. To average 100mph cross country is completely absurd.