So I did 301.6 miles today. 25 miles on narrow gravel mountain road, about 175 more on twisty two lane paved mountain roads and the rest on interstate.
I stopped at 212.6 miles (just as the last segment of the fuel gauge started to flash) and got 2.731 gallons for 77.84 mpg. I didn't top off the tank after the last 89 miles home.
I had the boxes on but nothing in them. Total elapsed time was 7:45, but 45 min of that was lunch.