What is the best size prop for a 1989 Sport 20 with 200hp blackmax merc?

[Destroyer]

Originally Posted by CharlestonSC

I have a 14.25x19 SS now and it takes off quick and tops out at 43mph on GPS at 59K rpm.
Think of a 14.25x21 to give me a better gas mileage and more top speed to get me to my fishing holes faster.

A 21p (that's what I run on my 200 merc) might give you faster top end, depending on what your WOT revs read and what your engines factory recommendations are, but it will not give you better gas mileage. Just like a car, the faster you go the more gas you have to burn to achieve and keep that speed. Friction rules, and you cannot change the laws of physics.