You can get following savings:
- voice files about 40MB
- unused vehicle 10MB
- text files about 10MB

Next you can remove help files, no more needed, when you removed language text files. Go to \Garmin\Help directory and remove unneeded subdirectories. You can get additional 2MB.

You can clean track archive, on my nuvi there is subdirectory \Garmin\GPX\Archive. Nuvi gather current track in file Current.gpx. When it grows too big, old content is copied to Archive subdirectory and saved as 1.gpx, 2.gpx etc. You can copy archived tracks to PC and delete from nuvi. Archive directory can contain several MB of data.

Last, but not least you can change basemap in your nuvi. Basemap is contained in file gmapbmap.img in \Garmin or \.System directory. Current nuvis use basemap with DEM data, map has about 100MB. You can swap it with map without DEM, with size form 7 up to 30MB.

I recommend to make a full copy of nuvi content on PC before any attempt to remove files.