I frequently will use this to aide in determining offset....
II. Here is also a wheel offset calculator.....
http://www.1010tires.com/WheelOffsetCalculator.asp
Also another very good resource provided by Eli.....
http://www.willtheyfit.com/
III. Now that we know the above......say I am going to go from the rim I have now.....to a stock style 135. Lets just talk about the rears.
CURRENT: Enkei's are 18X8 with an ET +42 offset.
FUTURE: Style 135's are 18X8.5 (rear) with a ET +50 offset...
IV. Plugging the above values into the calculator in II, above.....it yields the following information:
Inner Clearance: >>>>>>> 14mm's less than my Enkei's from the inside of the wheel to the strut housing.
Outer Position: >>>>>>>>Retract by 2mm's (position of the outside edge of the wheel)
***So....what the above numbers are telling me (in IV, above) are.
I may need spacers, if I do not currently have 14mm's from the edge of the wheel to my strut brace.
Additionally...my tire will still be recessed 2mm's more inside of my wheel well. Close fitment can somewhat be taken care of through the use of a stretched tire, or by adjusting my camber.
EDIT: I will be updating this thread with any good info I find on wheel fitment. Please feel free to add any of the tools, you guys use, to ensure proper stock, or slammed fitment.