I don't believe it. If those figures were accurate - you've probably got a world-record breaker.
My 1985 Isuzu mini-truck with a 2.2 liter diesel gets 27-28 MPG.
I've driven and owned many 7.3 diesels in Ford trucks. The best MPG I ever got on a slow highway cruise (55-60 MPH) at a low altitude on a flat road with no wind was 19.5 MPG. That with a 5 speed OD trans and a 3.5 axle ratio.