Surprised no one else has mentioned... the 39mph avg is just that... an average. The article didn't really mention the routes or what exactly was meant by "driving no more than 14 hours a day", but I would not assume they got in the car in the morning, accelerated instantly to 39mph and then kept the speed constant for the next 14 hours til they stopped (instantly). I would guess they did not take freeways (straightest, fastest route is only ~1400 miles, not ~1630 miles according to Google maps) and that the 'driving time' included some rest stops / food stops along the way. I'm sure they didn't do 70mph, and they probably did drive relatively slow (45? 50?) most of the way, but it was probably within a reasonable range of the roads they were on.
Heck, I did ~2000 miles traveling 65 - 70mph and got 60mpg average... not all that hard in a Diesel. I just would have a tough time doing ~1600 miles on a tank since my tank is 3 gals less...
