Unless you've cached them, maps come down real-time via your data link so if your viewing your position on the map as you ride, you're using data. I don't know how much as I'm grandfathered in a unlimited data plan so never bothered to check. I don't think it's enormous though. Turn off email pushes and any other pushes. Traveling out of the country I just use a Garmin and keep the phone data turned off most of the time.