Wheel Offset
Use this calculator to determine the effect of changing your wheel width and offset.
First enter your current wheel width and offset.
Then enter your new wheel width and offset.
Next click the calculate button.
It will show the clearance between the inside of the wheel and the strut housing.
It will also show how far the outside of the wheel edge will extend or retract.
If you reduce the inner clearance too much or push the wheel out too far, the tire might rub or not fit at all.
http://www.1010tires.com/WheelOffsetCalculator.asp
and also
http://www.yokohamatire.com/utcustom.asp