i'm running 15x7 with 235/60-15 up front and 15x8 with 275/50-15 in the back. i did have to roll the fender lips in the rear to clear the 275's but otherwise no mods at all.
The cheapest and most common way to roll the fender lips is to take a wooden baseball bat and place it between the tire and the inner lip of the fender. Tilt the bat at an angle as that it rests on the tire and the fender and have someone push the car slowly so that the bat rolls with the tire. Put pressure on the bat so that it shapes the fender lip up and out at the car is moving. This wiil give your tires a little more clearance. Actually works pretty good without any notice metal damage.
I believe that autozone has a fender rolling tool that you just bold to your hub and rotate the hub to roll the fenders in their free loan-a-tool service. I have also read articles that say that if you want to save your paint, you need to use a heat gun on it and heat the fender to something like 120 degrees.
oh yea, and not to hijack but I am wondering how to measure backspacing. I am not to that point yet but is there a formula or something to figure out what I need?
235/60-15 on a 15x8 will fit and possibly fit on the car but you may have rubbing issues. even my 15x7 rims have occassional rubbing issues with the front valance but i don't think the wheel on the side that rubs is centered perfectly, too much positive caster, i think.