Garmin 500 and strava speed issues?

CyclingObsession Posts: 314
edited November 2012 in Workshop
I'm using the strava with speed sensor and its greät very accurate, however when I upload my route to strava on some segments it says I was only doing 3.7mph average and 18 when I'd been doing 20+ bit it does not reflect this on strava, I never had this issue when using the app? Has anyone else experienced this?