How much data will the app use to upload journey data?

Not very much. It depends on how much driving you do but on average it’s about 10MB per month (roughly the equivalent of downloading 3 songs). Monthly data allowances vary by package but typically start from 150MB per month. You can change this so, rather than sending over your phone provider’s data network, the app will only do this when you’re next connected to wi-fi (go to Settings in the app).