do you have a fuel flow meter? (flowscan etc) invaluable for running at best economy, boat that size maybe get used to about 1.3-1.8 mpg depending on speed and conditions. if engine is sound and tuned and you have right propset to hit recommended WOT rpm with normal load you probably can do much better ,or if you are be happy.
flowscan will normally pay for itself in a very short while with boat like that
and watching it will encourage you to try to drive at best cruise.
with 175 gallons of $4 fuel thats 700per tank.
if you can fine tune trim, speed,load, etc to get 10% better you will save
about $70 per tank, maybe even more.
worst economy is often from just above 5knots to about 16knots boat plowing water but not on plane.
make sure her bottom is clean, your arent over trimmed or under trimmed and know your fuel flow and calculate your economy continuosly.
example , there are many sellers and a several models.
http://www.defender.com search instruments for floscanCruisemaster Series 5500
under 300 shipped unless you have efi with return line