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I recently ran across these two sites that ROCK for figuring out tire sizes and wheel size and offsets. The first site is a simple sites but oh so powerful in being able to visualize what's going on and it gives you the difference in speedo speed that will occur and tells you how much more the tire will stick in or out etc. Very powerful and impressive site!
On this next site you can use it to look up some wheel specs as well. I can't find a lot of GM on there but for my import car I was able to find some factory wheel specs on there. Just posting it up for there's some good info on that site.