I don't know if this has been posted before on one of the other Wheel and Tire threads (there is to many to check), but here is a link to shown how changes to wheel width and offset, change inner tyre clearance to the strut And the position of the out side edge of the tyre.
http://www.1010tires.com/Tools/Wheel-Offset-Calculator
http://www.1010tires.com/Tools/Wheel-Offset-Calculator