How would you be able to use a wider tire with and aftermarket 15x6.5 as opposed to the stock 15x6.5? That doesn't make sense.
If you have wider wheels, like the 17x7 slipknot has, then you need a slightly lower offset, like +48, to keep the steering point in the middle of the tire.
Here's an article you might want to read about that if you're curious.
http://www.sportcompactcarweb.com/ed...0205scc_techno