Changing a gear will produce what RPM change ?

So I know I/O?s have prop slip calculators and are generally regarded as a 1 inch change in pitch will produce a 200 RPM change. Is there any rules of thumb for V drives ? IE. If I change the gear in my V drive by 1 size all other things remaining the same what would the RPM and speed change be?

About 300 RPM

There are so many things that dictate the answers to your question. And yes, there are multiple answers, and quite often changes bring more questions than answers... To say you'll enjoy a 300 rpm increase by going to a "one step" lower gear may be correct, but not absolute..
There is a fine window of performance between gear, prop, engine, plate, and boat balance.. A lower gear ratio MAY put the engine in a better "happy place" performance wise, but turning the same prop a little faster may result in lift, nosing the boat over and making it run "wetter" with the same plate settings.. I guess what I'm trying to say is boat performance is a package deal.. Changing one thing can go either way. That IS NOT TO SAY you should change more than one thing at a time, just that there is no real rule of thumb what's going to happen...
The most important thing is NOTES. Notes with good data using GPS speeds and tattletale tachs. Notes with accurate plate settings recorded and the results obtained.. Notes about the prop, (and how far it is from the strut), the gears, and of course the tune up with atmospheric data.... AND, the all important "seat of pants" feeling. How did it "feel" coming out, in the midrange, and up top?? Did it stop pulling even though the Rs were still going up? Did it nose over at a certain RPM? Did it carry the nose throughout the trial?? Write it down, every little bit of input you can think of, and work from there...
Ray

with just a gear change, you get that all important "front of the pants" feeling too ! HA HA