As I recall, he gets 60 miles per gallon of gasoline, but he uses a second fuel source — hydrogen — which he doesn’t account for. If he used hydrogen for 99% of the power, he might be able to claim 1,000 miles per gallon of gasoline. But then people would question his results.
60 miles per gallon of diesel, using /two/ secondary sources of energy - the battery packs as well as the hydrogen, both of which aren’t properly accounted for, and even then, it’s a ‘guesstimate’ that he gets anywhere near that fuel economy.