... when it just simply sounds good, when you can tell the engine likes that RPM, it is happy ... I usually don't look at the Tach to pick the speed. Whatever this RPM, it is certainly lower than 80% of max, perhaps closer 70%. That's about as fast I like it. At that speed, we are probably about .8 gal/hr. When motor sailing or in no rush, we'll often run 1800 to 2200, and my max fully loaded is about 3400 (1986 Yanmar 4JHE with 4300 hrs.). Then we're more like .65 gal/hr. I use Rotella 15W40 and change the oil/filer ever 100 hrs.

Cheers, John