Most of DMD2 is offline. What costs data is map tiles, and after that group riding.

Rough figures to plan with:

• Group ride — around 10 MB per hour, a little more or less depending on how many riders are in the group. This is the constant drip: positions go up and come down for the whole ride.
• Satellite view — around 35 MB per hour, depending on how much map area you cover and how far you zoom.
• Hillshading — around 40 MB per hour, on the same basis.

Anything that draws tiles over the map belongs in the same bracket, and they stack: the national topo rasters, public GPS traces, the worldwide fire layers, any custom raster you add, and the weather overlays. Be aware that the wind and temperature overlays wash the whole view rather than just the areas with weather in them, so they fetch every tile on screen.

Small by comparison, and not worth worrying about: the country-specific modules and the HUB sync, which exchange short status messages every few minutes rather than images, and the home weather card, which refreshes a few times a day.

Two things that help. Tiles you have already seen are cached, so riding the same area again costs far less than the first pass. And everything else — downloading a map, in particular — happens only when you ask for it, and should be done on a decent Wi-Fi connection.

If you want the map to use no data at all, leave the Online Map Layers off. The offline vector map and the routing work with no signal.