I finally got the true GPH on the new diesel.
My wife and I ran the boat with full fuel, inshore/offshore tackle, bait, ice, drinks, food and clothes for a week of vacation. We ran the boat 92 miles from Freeport, NY to Montauk, NY @ 2800rpm the whole way averaging 20.5kts in a 2-4' quarter following sea.
I was please to find out that we are doing 6GPH at that RPM. On a calm day, 2800rpm gets us 21.5kts.
Also, something I never knew but someone told me and it appears to be true, boats run faster at the same RPMs in colder water. When I first launched the boat in February, I was getting better than 22kts @ 2800 and now that the bay temp is 70 I am getting 21.5kts at the same RPM. I guess it makes sense as colder water is denser and must give better traction.