We moved this over to non-Repair I/O Jacalore. Been a lot of discussion about this, but the fact is that this is more of a hull issue than an engine RPM issue. Generally speaking though these combos end up most efficient at around 30 MPH which is typically between 3000 and 3500 RPM. Some heavier boats end up being closer to 4000 RPM for an efficient cruise and some faster combos may be down at 2500. It's more about getting the hull running clean, and again that's usually around 30 MPH. Take a cruise over to Boattest.com and study similar boats. You'll start to see what I am talking about.