The fuel comsumption you describe given the age of the motor, weight of the boat, and running at WOT does not sound unreasonable. I had a 16' Glastron with a 1977 85hp Evinrude and got about 4 mpg at cruising speed. A friend had a 65hp Mercury on a trihull just a little larger than the one you describe and I could go twice as far as he could on the same amount of fuel.