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GPS Not Accurate

I've had my Garmin 645 (no music) since July. Within the last month, the GPS has been way off. 5 miles into my 22 mile run last week, I went from an 8:20 pace to a 6:45 pace to a 6:30 pace to a 12:00 min pace to a 16 min pace, etc. Super frustrating! I have had Garmin watches for over a decade, so I know these things happen, but this is happening every single run. I've tried updating the watch and resetting. Nothing seems to work. Anyone else experiencing these same issues? Any recommendations on how to fix this? Thank you!

  • How is the gps trace on your runs afterwards? if it is really bad, this may be a defective chip.
    I've found the 645 GPS to be okay-ish but consistently (in my case) cuts out 5% or so of the total distance. I use a foot pod so as to be able to rely on a sane instant pace measurements.

  • Valencia marathon, 43.18 km.

  • In my 645 Music I have seen the same rapid changes in pace too, mostly from 7:00 to 10:00 min/mile paces within 30 seconds of running. I don't think it is firmware-related since I have been seeing it since July 2019.

    My observation was that the slow pace was related to the mild tree cover on my rail trail runs. When I get back under clear sky the pace quickens significantly. 

    I've been in touch with Garmin support, who suggested that I remove CIQ data fields (I was using Dozen Run and Race Screen). No luck. They are now offering to exchange the watch. I'll probably do it but I wonder if this is just a 645 design flaw and I'll see the same issues.

    On a related topic, one of the Garmin support people offered to loan me a Garmin foot pod. I already had one, but I still see the same issues. Garmin support told me that the accelerometer/foot pod input is used when the GPS drops completely, but I do not see any error messages that GPS is lost. Does anyone know how using a foot pod would help (unless I use the foot pod as the sole pace and distance input)?

  • It works mainly, but not very accurate in towns and deep forests. Another marathon three months back, and my 645 measured ~44 km. Never had  such issues with my previous Garmin 210.