Was doing legit 40 plus going down Hurricane Ridge with road closed to vehicles. Garmin Edge 500 recorded 60 plus section.
https://www.strava.com/activities/35...ysis/5092/5200
Turns out to be due to lost wheel magnet so Garmin edge converted to GPS for speed. Went thru tunnel, lost sat. and Garmin just updated when sat picked up. So there can be other technical reasons for some. BTW at 75 that was by far the most exciting ride of my life. I will never forget that one ride.