Re:Pitch and RPMs.. on my I/O ( a 5.7L, 260 HP MERC MIE (2bbl) in front of a Volvo 280 drive), the MERC factory manual says... operating range 4200 to 4600. Just so long as I am within the range, the engine will be happy. However, within that range, there are differences in operating characteristics. If for example, a 19 pitch prop would get me 4550 RPM and a 21 would get me 4250. Then I would be able to get a bit more top speed and a bit better fuel consumption.
Why is this... Assuming a 1.66:1 reduction and 14% slip at 4250 and 15% slip at 4550, the 19 pitch at 4550 would get me about 42.4 MPH, while the 21 at 4250 would get me about 42.3 MPH.
So where does the improvement in fuel comsumption come from?
On 4 stroke engines there is a parameter called (MEP) Mean Effective Pressure ( internal cylinder). As the MEP increases, the engine is working more efficiently and fuel economy improves somewhat. Increasing the prop pitch to 21 causes the engine to "work" harder ( again within design limits) and the MEP increases.

Some folks think that the prop that gets them the highest RPMS within limits is the best...not always so. Sometimes, top speed actually drops at the higher pitch with the engine still within operating range, then you get to choose betwen a bit more speed and a bit better efficiency.