Re: Mpg 3ltr
that motor can burn as much as 7.5 gallons per hour at full load
It will easily be more than that at full load. A good conservative rule of thumb is to figure that a motor will burn 1 GPH for every 10 HP at full load. So a 140 HP 3.0L will typically be somewhere in the neighborhood of 14 GPH UNDER FULL LOAD. At best cruise, a 3.0L will usually be burning about 5 GPH and running somewhere between 25-30 MPH.
As far as the cruising MPG the original poster asked about, for a 3.0L it will typically vary between 4 to 6 MPG depending on the boat. That's why Bondo stated that we didn't have enough info and I suggested he compare tests on similar boats. For example, the test below is on an 18' Stingray, you can see that it burns about 12 GPH at WOT (wide open throttle) and gets a little less than 6 MPG at cruise with only one person on board and less than half a tank of fuel.
Best thing to do is to pick a couple of boats similar to yours and look at the tests to get an idea of the MPG you'll see at cruise.
