I had a '78 with a Cummins BTA, and generally ran slow - 7 knots or so - to get good FE. Just dug out my log from doing the Loop on that boat a few years ago,
Distance: 5,815 NM
Motor hours: 893
Generator hours: 299
Fuel used: 1,523 gallons; 5,757 l
GPH: 1.7
MPG: 4.4
I'd suggest that as a...