I have a Trek Incite (8i or 9i - wireless no cadence) and I have the exact same problem. I did a roll out calibration when I installed it. The trip distance is perfect, the average speed is right, but the Max Speed will read 64mph or 77 mph, always high.
The other day on my commute home I stopped on the top of the first hill and it showed a max speed of 38 MPH. I maybe broke 22mph on the flat on the before climbing the hill....