You can read all about that GPS here: that Street Pilot III is incompatable with all of the new NT formatted maps so the latest (and last) maps that GPS can possibly use (if you are in the USA) is named "City Navigator North America 2009".These are the last Non-NT maps produced for North America and there have been 7 map updates since their release in 2008 so by todays standards they are still very old.If making a cross country trip, you would have to selectively copy segments that cover your intended route; hopefully all the segments needed will fit on that data card.
At least the current series of handheld GPSrs such as the GPSMAP 60Cx and e Trex Legend Cx retains NEMA support.
The GPSMAP 60Cx could be used as an automotive GPSr in a pinch, but the nuvi is arguably a better design for automotive use. I'm a nice guy, but being that this is a buddy's GPS, I don't want to get in deeper than I would if it were mine.
The possible good news is that I own those 2009 maps and will never use them again so if you feel ambitious and want to play with that unit I'd be happy to simply give the maps to you. The caveat is that the map tiles (smallest map segments you can get from those maps) that can be selected for loading onto the data card might be to large to even fit on the data card (cards came in multiple sizes).
Over the years, Garmin made a lot of changes to the sizes of the map segments as the different GPS models came out with more internal memory. You would have to purchase City Navigator Version 8 dated 2006 and get an unlock code that would permit the map segments to be copied to the proprietary data card used in that particular GPSr.
If memory serves me correctly, the only way to get detailed maps into that GPS is on one of the Data Cards.
The GPS contains only a built in "Basemap" with no street level detail. They are proprietary Garmin cards and very expensive, even today.He would love to upgrade to a newer unit but the SPIII supports NMEA output which he uses for Amateur Radio APRS.None of the current auto GPS's support NMEA directly.There are some adapter cables available that allow conversion from Garmin's proprietary Fleet Management Protocol to NMEA but support is limited.It is another case of Garmin removing features from newer units.I had no idea what I was missing until my wife bought me a nüvi a couple of years ago.