That's the same size prop I ran on my 115; it was an OMC SST and was perfect for heavy loads - used to turn up to about 5400-5500 with a 2:1 gear ratio. Your boat is probably lighter and that more modern motor is rated at the prop instead of the powerhead, so if your gear ratio is the same I would think you could turn a little more rpm. If it overrevs, a good prop shop could put a little more cup in the blades to knock it down by 100-200 rpm which would also help efficiency, but you need to the motor height right and check the tach before fooling with the prop.
I used to figure about 8 gallons/hr at 4500 rpm and 20 kts. We didn't stop much on those trips, running for at least 3 hours at a stretch, so that may be why my gas mileage was a little better than Gillie's. Stopping often and climbing up on plane again burns lotsa gas!
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