I will be on the stock 18x8 rims and i'm purchasing Dunlop Direzza Z2 Star Specs. From what i've gathered, going from my current 225/45 to the 245/40 will result in a slight loss in responsiveness but more grip overall, and higher acceleration Gearing wise. These will be for autocross so grip is my main concern and i kinda wanted a fatter tire to cover more surface area. All last season i auto crossed a Goodyear all season tire, So as far as me noticing this decrease in responsive turn in, i doubt i will. I'm sure i will only see great improvements on every front (except tread life). But is it reasonable to go to the 245? Or should i stick to the 225. I like the idea of a fatter tire covering more ground for more all around grip.

 

I'm not within your class but just to help with my experience with wider tires on the 8. I was in regional autocross last year in the STX class on 17x9 +45 Enkei RPF1s and with 255/40/17 Toyo R1Rs and let me tell you the grip increase will make you insanely faster! The confidence you get pushing the car in an autocross setting with such a wide tire is pretty awesome! So my suggestion would be to go as wide as you can but I'm pretty competitive and my 8 is just a toy now so everyone has their own uses for their 8.

 

Go with the 245/40. The 45 would be too tall. I run STX in San Diego and you can fit a 245 on a stock 8" rim no problem. I wouldn't go wider than that though unless you got wider wheels. Kumho XS is a great tire for Grip and longevity. If you want out right grip the Hankook RS3 or looks like the new Bridgestone RE71 are the way to go.