Coordinate system doesn't contain values in the range -180 to 180 and -90 to 90

Question

I'm creating an android version of a PC program (I've contacted the company who owns the PC program, so it's legal). The program is in the core a GPS, but is used to navigate pre-defined routes for heavyweight transports, because they are not allowed to pass some bridges. However, that isn't very important for my problem. The program reads the coordinates from text files, so my app needs to do that too, but those coordinates are not in the range -180 to 180 and -90 to 90. So I can't really understand them. For example, one coordinate has these values:

X: 6603939,57000491
Y: 575841,753705909

I need to convert these coordinates to WGS84 coordinates (lon and lat) in order to be able to use them for what I want.

Answer 1

I recommend you to have a look at the webpage epsg.io. There you can search also for regions to see which EPSG-codes are common for these regions. As you have written in your profile that you are from Sweden, I guess the coordinates are from Sweden,too. The best match i found so far was EPSG 3006 but i guess you flipped the x and y -values, is that possible?

if you flip x/y-Values and use EPSG3006 you end up in sweden in the middle of a road: http://epsg.io/3006/map