Don,
While testing props recently, we noticed a difference of approx. 1.3 MPH difference between the Merc Monitor (showing faster) and the read out on my Lowrance HDS 8 GPS. I assume the GPS is the more accurate between the two.
What is your opinion?

BTW- my motor has a little over 20 hrs on it (didn't notice the last time I ran it) I will be bringing it by in a couple of weeks for its service. Maybe then you can see if there is anything affecting the RPM issue.