I did some interstate miles today and check speed vs RPM ... With the speedo set at a steady 76 MPH it was indicating 4100 ... On my 2009 the difference between the speedo and the MPH indicatesd on the GPS was 3 or 4 MPH, on my WetHead the difference is only 2 MPH ... So I set 76 MPH using the GPS readout And I was running at 4250 which is a hash mark on the Tach. Hope that helps.
Ditto, exactly the same difference between the GPS and speedometer at high speeds: with speedometer 2mph higher than the GPS. It's 1 mph at city speeds.