You don't *need* data for phone apps that have offline maps (just download them at home like you would when updating a traditional portable GPS). There's no point buying more hardware (portable GPS) if you already have a touch screen device with a GPS chip inside. Just download an offline navigation app (I linked to one in my last post), buy the maps (unlimited free upgrades), download them, and you're set. Get a cheapo universal phone holder for your car if you need one and use the car charger.
Having said that, I'd still prefer the in-car integration which is why I have the RNS510 (amongst other reasons). But I have 1GB data with Telstra 4G on my phone which is a good supplement. I can check how much data each app uses on my phone, so far Google Maps as used 17.1mb in the last 27 days (I have live traffic conditions turned on which would be the main reason for this amount since I have most of S.E. Melbourne stored offline). By way of comparison I've used 200mb downloading podcasts to listen to on the train, 73mb of YouTube streaming, 57mb of Internet Radio, and 40mb of general Internet browsing.
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