thank youthis is a great question! how ever your missing a very important measurement; offset. Wheel Offset Calculator | 1010Tires.com Discount Online Tire and Wheel Store
I have had a friend of mine run 18 x 8 however the offset was incorrect and and the wheel would touch the strut. Here is a tire size calculator( Tire Size Calculator ), the two links should be more than enough to help figure out your issue and if you would ever like to go bigger!