With a std 14x18 prop powerslot setup engine rpm should be actual MPH, give or take a few. Ie 3600 RPM =36mph. Sounds like you need to add some more cup to your prop. Over time as props are overhauled they lose diameter, so you might need to add 1/2" pitcht or so. A good shop should be able to determine what they need to do based on current performance and what you want.
1988 PS190 Powerslot, INDMAR 351W, Edelbrock Intake, MSD Ignition,