The most fuel efficient speed will not necessarily be the slowest planing speed.
One of those fuel flow meters and a GPS is about the only way to get it real close and not necessarily will the least fuel consumption be the most fuel efficient.
The top numbers is gallons of fuel used per hour, the left side numbers is the speed of the boat, all the other numbers in the chart are miles per gallon.
And I haven't taken the time to check all of this on my boat yet. So I have no idea what the best speed or rpm is to be the most efficient.