If your in offline mode its not supposed to use any data. Streaming mode (online) uses tons of data. I have it on my iPod (not iphone) in offline mode. You also need to actually download and sync playlist that you want available in offline mode, not just hit offline mode. (Read the instructions on the spotify
website). To play it, you need to get it started before you plug it in and play it. Since spotify is a 3rd party app, and not the main music player, the playlist wont be recognized by the car so you have to select the playlist on your phone but skip and back controls are still available by steering wheel controls.