I went out yesterday and had a nice sail for an hour or so. When I came in I noticed my GPS said 28kts which I was stoked about as it would have been a new PB for me.

I then loaded my file onto KA72.com and came up with a 2 sec result of 22.15. I was understandably disappointed as I was sure I was going a lot faster. Session results here: www.gpsteamchallenge.com.au/sailor_session/show?date=2014-02-23&team=9

I then switched on the GPS and sure enough I saw a max speed of 28.49Kts:
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Any ideas what happened? I know I never leave it switched on for the drive home. None of my tracks have ever had speeds near 28Kts when I upload to KA72. But it seems to me that my GPS thinks it has gone over 28 kts.

That seems to have been a different session to yesterdays: Here's yesterdays track: http://www.ka72.com/viewge.aspx?fid=58081&tid=7

Also here's a link to all the datalogger information: www.dropbox.com/s/mhi3pwlrosh8s9n/ADAM%20HATFIELD_933000190_20140223_123839_DLG.SBP

None of those seem to have any results close to 28kts. Here's what Ka72 reports: anon.ka72.com/TrackRecalc.aspx?fid=58081

Had a look and it is a spike (skipped a second). However the track in realspeed does show what appears to be a valid result of 24.89 but with only 6 satellites which may be why KA72 is not showing it. You want 8+ ideally
Have a look at where you are wearing the gps and make sure that it does not go under your arm or get blocked by something you are wearing.

Bugger, I've had this vague hope that my GPS is faulty and I am actually quicker than it is showing. No such luck.

I wear it on my arm so I would have thought it would have good coverage. I'll make sure i wear it somewhere it can't be missed.

Thanks for taking a look.

It probably means you didn't hold the 28.49 for 2 sec's, that was your max speed but not for long enough.