I have a 5000 pound fin keeler, a Haida 26, with a Honda 8hp 4 stroke. It has a 21 foot waterline, and a calculated hull speed of around 6 knots. I normally keep the RPMS a little low, and go 4 knots, just starting to push a noticeable wake. This keeps me going at less than a gallon per hour.
I also motored this same boat with a Nissan 5hp 2 stroke at around 3 knots and less than half throttle, this didn't even put out a noticeable wake, and was painfully slow, but we used barely any gas at all, probably less than a half gallon per hour.
Wave making resistance - Wikipedia, the free encyclopedia
Your waterline length is going to be an important factor in determining how efficiently she will go through the water. A longer waterline means a higher hull speed.
Also, a high thrust prop will waste less gas than the stock prop.
I made the mistake of running my outboard at very low rpm for around 8 hours, and fouled the plugs, so keep that in mind, occasionally you should clear out the carbon by revving her up.