I don't know.
I do know they made great use of the "
Super Atkinson Highway Mode" sweet spot. As listed in that discussion, this spot is 50-65 mph.
14 days, 7500 miles = 535 miles / day
12-14 hours a day gives 45 mph average, including stops. The recorded stops for fillups and for state line crossings took a bit of time, I understand. I'd say their actually driving speed was more like 60 mph, then.