Which file do I use? First step: determine the size of map set you want. If you have a 512MB card or want to use up ~500MB of space on your device, look at the files with names starting with "500MB". These files should not _quite_ fill up a 512MB card. This is so that you have room for a bit more stuff, like track logs. I have a 4GB card, which is large enough to hold the entire US. There's only one "4000MB" file, so I'm done. If you are using a smaller card, you next need to consider your location so that you can download a piece of the map. I've cut them up into longitude swaths. Let's say I have a 1GB card, so I'm looking at the "1000MB" maps. I'm at about -122 longitude. I have three choices (in the 04/30/2010 set): [ ] 1000MB-lon_-84.20_to_153.85.gmapsupp.img 02-May-2010 15:48 1.0G [ ] 1000MB-lon_-96.86_to_-82.66.gmapsupp.img 02-May-2010 15:47 1.0G [ ] 1000MB-lon_-165.45_to_-94.83.gmapsupp.img 02-May-2010 15:47 1.0G -122 is between -165.45 and -94.83, but not in any of the others. So, I would pick: "1000MB-lon_-165.45_to_-94.83.gmapsupp.img" You'll also notice that the maps overlap a bit. Let's say that you are unlucky and you sit right at the point where I picked to chop up the "2000MB" maps. Let's say that it it at the Indiana/Illinois border. Maybe you live in IN, but shop in IL. It would be really annoying to need two cards just for a trip to the store. So, I guarantee that the maps will overlap by at least a hundred miles or so. It should make picking a map a bit easier.