Positon formats to match a map

> They are using one like this: 4894498.69 N, 488808.43 E

The maps are using this format?  Sounds like UTM (but missing the UTM zone).  As GOF says, go read the Garmin manual; the manual is (for once) fairly clear on how to set datum and coordinate displays.

BTW, this is predominantly a UK-based site, so its members probably aren't too familiar with US mapping...  There are some US-based members, though.

Like the captain says it sounds like your projection is UTM.

For the datum: If it's a reasonably up to date map try using NAD83; otherwise if it's a older (pre-GPS era) map then you could try NAD27. 

In either case, the geodetic information should be shown somewhere on the legend/map border.  You'e looking for a 'Datum' (WGS84, NAD83, OSGB etc.) which is the reference point for 0E 0N, and a 'Projection' (UTM, MGRS, British Grid etc.) which is how a lat/long is converted to a easting/northing.

You'e looking for a 'Datum' (WGS84, NAD83, OSGB etc.) which is the reference point for 0E 0N

The datum also includes the local spheroid in use; WGS84 uses a global spheroid model.  OSGB36 uses the Airy spheroid, which is more accurate for GB.  I'm sure NAD83 includes a spheroid model which is more accurate for North America.

The reference isn't necessarily 0E 0N, but is appropriate to the local datum and projection.