here is a little calculator to play with. It really depends on the type of hull. You can underpower (or throttle down) and efficient displacement hull and get great results. Just like slowing down a car because of aero drag but it's much more drastic with water. A 30 foot displacement hull has a theoretical top speed of 9.2 knots if you put 81 horsepower to it, but to go 8.3 knots on the same boat you only need 38 HP. If 5.5 knots was enough it only takes 8 HP.
Now on a 20' planing hull it takes 210 HP to reach 35 knots. Dropping down to 75 HP will go 15.2 knots. Down below 40 HP it turns into a poor displacement hull design and goes slower on the same power as the bigger heavier 30' displacement hull.
once you know the horsepower required for what speed you can figure engine efficiency and get a MPG.
Boat Speed Calculator
I also plugged in my ship the USS Carl Vinson for fun. The US Navy always says 30+ knots, but if you put enough HP to that hull it is capable of 65 knots. That would require almost 1.8 million HP. Well it doesn't have that much but it does has an advertised 260,000 HP which would be good for 37 knots (which is pushing 43 mph). I can't confirm or deny that number but physics is physics.