Do you try activate a raw logging for position in Nuvi 255? My Nuvi 765T is not inferior to any non-professional GPS devices in accuracy in this mode. 2-3 meters is total error due to using of L1 GPS channel.
One exception I am using external Garmin marine antenna on car root for reduction of correlation with directivity pattern. In this case tracks is equal without difference - go forward or go back.
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