mariner tach question

[MH Hawker]

i recently bought and installed a little 1996 9.9 mariner on my 20 pontoon, and dont have much experance with mercury, on the first trip out i tested with a clamp on tach, it shows the WOT rpm as being 2500 but the boat is running along at 10 mph with gps, i found this very suprising , i was expecting around 5 mph , the comander 2000 bontrol box has wiring for a tach, and i am intersted in stalling a perment one, will it work , thier is a question in my mind that i was getting the corret rpm, to me it dosnt seam correct that i was getting that speed at 1/2 rpms , any thoughts would be apricated boat n geet is around 1500 lbs