hi everyone, I have a formula 312 with outdrive ratio 1.65 26 pitch I cruise at 36 40 mph at 3300-3600 rpm, I have inspected but outdrive and my gear starts to undo, I try to get this by putting a ratio of 1.50 instead of 1.65 and by lowering my pitch this can affect my cruising speed?

I don’t know if it will affect your cruise speed but expect to go down 4 inches of pitch.

I had a 312. You must have the 310hp 454's for that pitch prop (the 454 mags and 502s) came with 30P props and 1.65 drives. It would seem to me going to a 1.50 drive and lower pitch prop would actually reduce cruise speed (but I don't know). What are you trying to accomplish?

Don't know what motors you have but, if your able to reach WOT with current props then your good.
Lowering the rpm's at cruise will not save any gas and will hurt the motor over time. A motor running easy at 3600 will use less fuel then a motor lugging at 3000. Like trying to drive up a steep hill at 25 mph in 4 gear

Switching from a 1.65 to a 1.50 and then lowering pitch will little to no effect on cruise or WOT speeds.
You would need to drop about 3" in pitch to maintain your current speed/rpm with the ratio change.