Got a request for wheel offset info, so I figure Ill just go on a roll here. Ive heard EVERY possible wheel related question I think, and 90% pertain to width and offset for some odd reason. Ill throw out the fundamentals here for ya, and you can work with it. Offset is simply the amount of spacing (for our cars in mm) from the center point of the wheel inside the barrel. Domestics just call it backspacing and do it in terms of inches. Offsets will be positive in most cases, so for xBs it is usually safe to assume if they dont put a "+" sign in front of the number, that it is still positive. The center point inside the barrel is referred to as "0" offset in which the rest is gauged. A positive offset in one in which the mounting surface of the rim (where itll sit flush to the hub) is closer to the front (outward part) of the rim, than the back. A negative offset, of course, is where that mounting surface is closer to the back of the rim.
I hope I havent lost anyone here. Now to start figuring out offsets, you have to have a basic knowledge of math (or a decent online calculator. i end up usually doing it in my head since i find it quicker and easier). In an inch, figure that is appx. 26 mm. When lookin at rim rize, you of course see width. To start to figure things out, do basic math.
If you have rim that is 17x8 25 offset (mine), how much more does it stick out than a rim that is 17x7 40 offset. Now just reason it out. Theres an inch difference between the two in width. Now in ALL cases, think of the rim as two halves. Half of the rim is inside, half of the rim is outside. Therefore, the rim will stick out 13 mm more than the previous rim, based on width alone. Now figure in that, PLUS the fact that the mounting surface is 15mm more inside the barrel, itll push it out that much more. Therefore, the rim is added up to be 28 mm closer to the outside of the fender than the previous rim. Now I know I may hvae lost someone here since Im not good at explaining, so heres a calculator that some may use.
To figure out inner clearance, you can do the same basic method. When figuring in the least amount of inner clearance, I use 7" wide and 42 offset as my basemark. (its ridiculously close to inner suspension).
Heres picture for those that arent following what offset really is yet:
Following? Now I know you may ask OMG OMG will this fit? Well to tell that, I need to know some things. Width of tire, offset, width of rim, rolled fenders, and drop that you run. Theres no exact perfect way to tell on ALL accounts. On most I can give a 99% guess, but some I cant. On 17x8 25 offset I was rubbing becausse of a huge drop. 15x8 0 offset might not because of a small drop. I can give a good guess for most. Offset will depend on what look you want to achieve (VIP being the most aggressive aside from widebody and flares).
It boils down to some basic math and some udnerstanding of how things adversely will add or subtract. Now I know I may have lost some of you on this, and I apologize. Im just trying to give a basic understanding on this. Worst comes to worst, people can continue PMing me and I can answer things. With that said, I hope this helps! (sorry for the long windness and confusion)