I use a speed and cadence sensor built into the bike now but even before that the Garmin average speed always got trimmed back by Strava. Track session last night my Garmin lost the first 50 minutes of the session - we're not allowed Garmins on view so I shoved it into my jersey pocket - got it out 50 mins into the session and it had switched off for some completely random reason - GRRRR. Switched it back on just in time for an end of session 36 lap race so at least I got that recorded.