prop size irwin 37cc 3200rpm

Hi, I have a 1980 MKV with a Perkins 4108 engine and gearbox with a 1.56 reduction. The manufacturer tells me I should cruise at 3200 rpm, which is about where I hit a comfortable cruising speed. My prop is 16 X 16 3 blade but when I run the numbers through the Vicprop prop calculator it tells...