I've had this problem forever but it's really starting to annoy me. I've been using it for cycling and on every one of my garmin connect downloads I'll get a random max speed for like 218MPH.
I took it running today and my max speed was 22mph - I wish.
I tried to search to see if this is a known issue but I couldn't find anything. And yes, my firmware is up to date.
Thanks.
http://connect.garmin.com/activity/136223711