The maths would be:
5800 engine rpm / 1.85 = 3135 prop rpm
3135 prop rpm x 21 inch pitch = 65838 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
65838/63360 = 1.039 miles per minute
1.039 x 60 = 62.35 theoretical mph with your set up at 5800 RPM, assuming nil prop slip.
If you are getting true 62 mph, then your prop slip is 0.35/62.35 = 0.56% : very good
Just as a comparison, the other day I was on a boat running at 5300 rpm on a 1.87:1 gearbox, 21" pitch prop and getting 46kt (roughly 50mph) Theoretical speed on those figures would be 56.4 mph, so prop slip would be 11.35%
Do I need to go back to the maths class or was there perhaps a 10kt current running with you?