Gop-Dogg

I currently have 235/40/18 Pirrelli P7000 mounted on 18" x 9" wheels (front) and 265/35/18 Dunlop Sp Sport FM901 mounted on 18" x 10" wheels (rear) on my M5 (The previous owner mixed wheels for some reason). These tires are about done, so I'm looking to replace them w/ Potenza S0-2 all around. However, as you can see from the pic below

<img src="http://pages.sbcglobal.net/gopdogg/_images/M5%20side%20Long.jpg">

the rear tires appear to be a size too narrow. What size tire should I get to fill out the wheel? Also, does it make sense to upgrade the front to 245/40/18 or 255/40/18 at the same time?

Polar

it's gonna rub against the fender liner if you go for wider tires at the back.

Hunteh

Stock tires were 245 in the front and 275 in the back...