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iTrader: (3)

I am looking to get a set of used wheels, but am completely new to this whole wheel sizing thing. I have an 07 IS350, no plans to drop the car. I am thinking 19 X 8.5 Front and 19 X 9.5 Rear as 20s just seem too big. The problem I am having is with offsets. I have no idea what I need. I am looking at other forums for other makes (G35, BMW, etc) except I have no idea what offsets would work, esp if I end up with a 19 X 9 in the front and a 19 X 10 in the rear.

Any help would be GREATLY appreciated.