sorry i know this question probably has been asked 30 times but what is the fattest tire you can put on a-spec wheels

 

Use the 1010 tire calculator to figure out the correct diameters, approximately 26.7". You are somewhat limited by the 8 inch width. 245/45-18s have a minimum wheel width requirement of 7.5 inches, but as you start going up in width and down in profile, you rapidly exceed 8 inch widths. So the next sweet spot might be along the lines of 275/35-18, but you will need a 9 inch minimum width for that.

Contact slawek65 who has experimented with much wider tires, but he also has wider wheels.