As others have mentioned I don't think the formula holds true in the real world. Our 62hp Volvo (non turbo) has averaged 1.0 gph consistently over the years (1988 model). At 6.5 knots this equates to - ta dah! -6.5 mpg. A guy in our marina just had a new 32' aluminum catamaran power boat built, he burns app. 25 gph (two 370hp Volvo diesels, turbo charged+intercooled+electronic) and cruises at 30 knots so he is getting 1.2 mpg (and spending $75/hour),ouch!
SV Laurie Anne
1988 Brewer 40 Pilothouse