Your 5km time would suggest that there is a couple of seconds still available. Conversely, to get a sub 25 min 5 km, you need to find about 5 seconds for your 1k efforts.
I am off to a more reliable 1k next week. I am finding the garmin somewhat inconsistent.
Maybe you should use Julie's garmin. ;-)
Get one that indicates 1 km at 950m, that will get your time down