Very high idle/hard to find neutral

[Destroyer]

Originally Posted by Duke01

39 mph yesterday, measured via GPS. 40+ gallons of gas. One person on board. standard 3 blade alum prop. 1980 Merc 200HP.

Speed is relevant to a number of things. Pitch of the prop, number of blades, cupping, prop material, and a host of other things on just the prop alone. Weather, wind and sea conditions also play a huge part in your speed.
Having said that, 39 does seem a little slow. I run a 1993 200hp Merc and regularly see mid 40's.