It's hard to state slip to such an accurate number anyway, coz the only thing you can be sure of, is the 1.87:1 gear ratio.
Prop pitch could easily be a little different to what's stamped on it, or what the prop shop has told you, the Tacho is unlikely to be much better than 10% accurate, so RPM is unsure, and the GPS reading is only the peak reading it saw, so it coulda gone a bit quicker than that, or a bit slower (tidal flow is another factor if using GPS for speed readings)
So, whilst I don't doubt for one second it runs well into the 80's, slip factor is a bit of a grey area.
This is of course, just MHO
"I Agree with everything you say really!" - John Cooke to Jon Fuller - 26-01-2013