Was asked by my family if I can update their Garmin device, so I searched the net how to do it without getting into buying much software and possibly work with free maps.
Get the maps
Go to garmin.openstreetmap.nl and select what data you want on your map.
Start by choosing a type of map – for example “Generic Routable” (seems good for this model of Garmin), then select a predefined country from a dropdown menu (for example in a dropdown “Europe” you can choose Austria, or Slovenia).
If you want more territories add some additional tiles, click “Enable manual selection” checkbox and click around the map to select or deselect tiles. You can also click+drag to select multiple tiles. Once you are done, add your email address above the map and click “Build my maps” and follow instructions.
When you open the link in the second email, download the “Compressed file that contains a single image that can be placed directly onto the SD-card of the GPS.“, usually called something like osm_generic_new_gmapsupp.zip.
Add the maps to your Garmin
Plug your Garmin Nüvi 255 to your computer using a simple mini-usb cable (it looks like this).
Wait till it ‘boots’ – wait for progress bar and a beep.
After that there should appear two possibilities to mount in your Nautilus. One of them is the Garmin device itself and the other is the microSD card plugged into the Garmin. If names of devices don’t give enough clues about which one is which one, you can recognise them by their contents if you mount them. The microSD card will have only one folder called ‘garmin’ while Garmin device will have more folders and files. Mount the microSD card. (Alternatively, you can take the microSD card out of Garmin and mount it on computer using an SD card reader).
Backup (copy) the file or the ‘garmin’ folder from microSD card onto your computer just in case something goes wrong.
Unzip the contents of the zip file you downloaded from OpenStreetMap – this will give you a single file: gmapsupp.img. Copy it to the microSD card into folder ‘garmin’, overwriting the file that is there.
Wait for the transfer to complete and properly unmount the microSD card and the Garmin device. Once you are sure they are unmounted you can unplug the Garmin device from your computer.
That’s all. When Garmin starts it will include the new data together with old data for areas that you didn’t choose (or so it seems – untested yet). If you want to use just the new map, and not the old data (or just to test if the new map is installed), go to Tools > Settings > Map > Map Info where you can turn on and off maps as desired.
Alternative method: replace the maps
The above method seems to “add” new data for selected areas. In other words, what you put on the microSD card is then used in addition to the maps that are already on the device.
It seems that it is possible to replace the maps that are already on the device by actually replacing gmapprom.img file on folder ‘Garmin’ on the device (not on microSD card!) with the contents of gmapsupp.img. That is, rename gmapsupp.img to gmapprom.img and with it overwrite the file on the device (do not forget to backup anything you will override).