Found out I already had a
TuneLink 2.1 Amp Charger. Received my iFrogz USB cable today (in yellow to match my phone of course). Just did a quick test with GPS and testing the units ability to charge. With GPS working and a Podcast playing simultaneously the unit charged rapidly from 93 to 100% and maintained the charge at 100% during the duration of the drive (albeit short drive to the strip mall and back home). I canceled the Scosche adapter before it shipped from Amazon, no sense having two 2.1 Amp car adapters.
The iFrogz cable was clearly listed as USB 2.0 but did indicate it was for both data and charging. There was no amperage rating so I can't be sure if the cable has anything to do with it. I can add that my old USB cable I had in the car did not fit snugly into the Lumia 920 but the iFrogz correctly.
I'm expecting a car dock tomorrow so I'll try to see if I can find any differences in charging rates from one or two other USB cables I have laying around.
More to come.
BTW this is the car dock I decided on:
Mountek nGroove Universal CD Slot Mount Was going to go for a far more expensive solution but I'm hopeful my McGuyvered solution will suffice.