Hi Welcome aboard!
Each boat has their own ideal speed with regards to fuel consumption.
e.g the type II's are most fuel efficient at 6 knots.
What you gotsta do is play with different speeds and then check with your navigator max range at current speed.
most boats are at their best in regards to fuel consumption at a speed which is between your ahead slow setting, and your 1/3 setting. A speed between there will get you best fuel rates.
Have a play.
Sink plenty!
