235/40 is a little larger than OE 255/35 diameter. 245 puts you in a weird place where a 40 series would be way too large, and a 35 series would be undersized a bit. I like how the 235/40 fills the wheel well a little better and may try to fit 245/40 down the road, but I'm already fighting some rubbing so...
Here's a calculator that I use
http://www.rimsntires.com/specspro.jsp
Edit: I searched around a bit and it looks like some people have run 245/40r18 without issue, so I'll definitely be looking into that for my next set of tires