Prop Help

[Jromero4]

I have a 2015, 2104 frontier with a 2015 175hp Suzuki 4 stroke. My prop is a Suzuki 3x16x21.5 R. With 2 people, fishing gear, small ice chest, 3/4 tank fuel and whatever else someone usually has when fishing I can turn around 5100-5300 RPMs and top out around 47-48 mph. The guy I bought the boat said he would turn around the same RPMs but for would do 56-57 mph.
My question is, why isn?t the boat performing like that anymore? And if you have the same boat what size prop are you running?
What are my options? Thanks in advance.

[MattMigues]

The operating range is 5500-6100 rpm's. I personally think you should be turning a 19 pitch prop. which should be a gain of rougly 500rpm maybe even an 18 pitch. but either way I can't see you gain 8-10mph. Also was he checking mph on gps or speedometer installed gauge?

[B Shark]

I love you, the check is in the mail, my boat goes 65 miles an hour.