That's a bunch of crap. No 160hp diesel burns 1.6gph at cruise. Do a simple calculation of rated hp x 3% and you will get pretty close to gph at cruise on a diesel. For a 4 stroke ob or ib use rated hp x 5%.
160hp x 3% =4.8gph each or 10gph total at 30 mph he nets 3mpg. Using his gph numbers he is burning total of 3.2gph at 30 mph for a net of 9.37 mpg.
PS at WOT diesels burn about 5% of rated hp and carbed 2 stroke outboards burn about 10% of rated hp at wot.
Also calling bs on 40kt top. That's my rant for the day!
I also know this because Mastry Marine in Tampa ran a test on a 23 seacraft with a 4cyl yanmar 230 and a bravo 3 and it burned between 6-7 1/2 gph at cruise netting about 3.2 to 3.8 mpg. FYI top speed was 38kts. For $23,000 installed it was out of my price range so I went with twin 4 strokes and net 3mpg at 25.5 kts. The diesel did not present big enough diffenence for me.