The best way to find out is to find out what the speed for best economy.
I have a 310 with two mercruisers and my speed for best economy is at 3100 rpm which gives me about 18knots.
Typically it is the top of your power curve ... that is the point were you get the most horse power with the least amount of effort (gasoline).
34 year old CLASSIC 300 DA, towed almost anywhere behind the Duramax Dually Crew Cab.
Engines
16 cyl, 700 cu. in./Alpha I's
All of the boat tests start at 1,000 rpm. The best mpg is usually at 1,000 and again around 3,300 - 3,500 rpm according to those test. Honestly, the BEST is the lowest idle you can maintain BUT, what's the fun in that? It takes forever to get anywhere and is like watching grass grow. It's just painful! If your impellars aren't new, you may overheat at idle too. If you are worried about mpg, toss everything overboard, wives, children, empty the water tanks, throw the dog overboard, get rid of the mother in law and father in law, get rid of the canvas, get rid of the food, dishes, towels, fishing gear, dinghy, outboard, blankets, mattresses, electronics, generator and anything else that isn't locked down. Don't forget those hefty spare batteries and CRT TV's! You won't need those to achieve your best mpg!!