Here attached the details of a run where Garmin assigned a record 4:58/km.
I can't find where this record was possible. I thought it was from "GAP average" but I understood Garmin does not take GAP data for personal records.
Furthermore, there's a huge difference somewhere from pace (passo medio) and gap (GAP medio), see lap 4, from 4:34 to 5:13, very strange as the run was totally flat.... ;-)
Thanks!