18″ Xxr 006 Staggered Whats The Widest Tires I Can Put?

I have a set of 18 inch rims The width of the front is 8.5 and the back is 9.5 whats the largest width i can put
I was thinking
Front 225/45/18
Rear 255/40/18
Would that fit? and how much can i increase.