I have found a very good offset calculator You simply enter your current wheel width and offset into the table adn your planned rim width and offset and the calculator spits out how much extra they will stick out or retract on both sides depending on widths etc. A very handy tool http://www.1010tires.com/WheelOffsetCalculator.asp All wide body kit owners this is the one for you. '
I've posted this like 3 times now lol. Don't you read the forums? Also, read this on TT.net: http://twinturbo.net/ttnetfaq/FAQpages/tiresize.html