No need for an app. Go into settings, then into "general", there is a usage option. Hit that, you can reset it whenever. Turns out I've only used 1.5GB the entire time I've had my iPhone haha

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth