Dmanj, that is fantastic! Now I'm off to find the factory wheel specs... Thanks!
Edit - For people following this thread in the future, here's what I've learned:
For a 2007 G6 GT with stock 18x7 rims, the bolt pattern is 5x110, and the offset is 41mm.
The rims I'm looking at are 18x8 with a 40mm offset. According to the calculator on 1010Tires, the new wheels will extend 12mm inward, and 14mm outward. This is exactly what I was hoping for: The extra width is just about centered, so the angular load on the bearings & such should be about the same as factory spec. Perfect!
Note to self: Insert witty phrase about my '07 G6 GT Convertible here, if I ever think of one.
Last edited by Indiana Zones; 05-10-2013 at 08:01 PM.