From what I can gather Garmin are using decimal Lat & Lon not minutes and seconds. So 54deg 59min might read closer to 54.97deg
Have a look at this page and see if this will help:
Have had an instance where a guy "lost" his broken down Ford F250 in the middle of the Desert because he had these confused was about 5Mi away from where his car was!
If you can UTM is much more accurate because it is almost impossible to get this part wrong.