Pitch/speed calculator How does your rig stack up?

Play with this have fun...

http://www.go-fast.com/Prop_Slip_Calculator.htm

My motor 2006 yamaha 90 2stroke. How do I get gear ratio?

I don't check this thread very often so I'm sorry it's been a long time since you posted your question... The 90 Yamaha has a 2 to 1 gear ratio.