j16 high thrust prop

I have a 2005 CS J16 with a high thrust 2005 Yamaha 25 HP 4 stroke T25TLRD motor. I am using a high thrust 12 1/2 x 9 prop and getting 18 MPH at wot. Others with a 25hp motor 2 or 4 stroke are obtaining 28 to 30 MPH. They use up to a Rascal 9x13 prop. My question is with a lower gear ratio on the high trrust what prop should I run with. I am struggling to purchase a tachometer because I do not know the poles required. Thanks, Marty

[boats.net]

The T25 requires a tach with a 12 pole alternator setting and has an operating range of 5000-6000 rpm's. You can use a different other than the high thrust prop but need to find out what rpm,s it's turning now.