I'm grandfathered into the old unlimited data plan so I've never really payed attention. I think it says it updates the traffic every 15 min so I doubt it's pulling much. The map is stored locally so no data there.
I got the TomTom app as well and really like it. One big point for me was the app will work without cell or 3G/Net connection. As long as the GPS can pick up a signal it's all good.
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