After not using it for a long time, I activated a new data plan on my iPad and have been using it in the car with my app for the past few days. This is not very scientific, but after about four hours the app used 430MB of data. Perhaps 67% of the usage was slow driving on unpaved roads with maybe 20% at 50mph on paved roads.
So that suggests around 100MB/hour data usage, but there are some important caveats. First, I was using 3d maps almost all of the time. I believe these maps use twice as much data because you are loading a regular map along with a map of the terrain. Also, this is a tablet with a larger screen than a phone, so I'd expect it to use more data to fill the larger area. A car is going to use more data than hiking on foot, since you cover more area and more tiles need to be downloaded to update the map. Fast driving will use more data, but the zoom level of the map also figures into this. If you zoom way in, a lot more tiles need to be loaded.
So, you might use considerably less data while hiking with a phone and using a regular (not 3d) map, maybe something like 30MB/hour? Would be interested to hear experiences from other mobile users!
So that suggests around 100MB/hour data usage, but there are some important caveats. First, I was using 3d maps almost all of the time. I believe these maps use twice as much data because you are loading a regular map along with a map of the terrain. Also, this is a tablet with a larger screen than a phone, so I'd expect it to use more data to fill the larger area. A car is going to use more data than hiking on foot, since you cover more area and more tiles need to be downloaded to update the map. Fast driving will use more data, but the zoom level of the map also figures into this. If you zoom way in, a lot more tiles need to be loaded.
So, you might use considerably less data while hiking with a phone and using a regular (not 3d) map, maybe something like 30MB/hour? Would be interested to hear experiences from other mobile users!