I attached this link for anyone who has questions about various offsets, how much clearance they would need, how tire size affects, etc.
Just type in the before and after info. and view the calculations. For a P11 G20t, we run a stock size of 195/60 15, on a 15x6 rim w/ 45mm offset.
G20 base is 195/65 tire same rim and offset. Hope it helps.
http://www.tolan-hoechst.com/cars/tirecalc.htm