245/40/18 vs. 255/40/18 (OEM 18" Rears)
For whoever is wondering, the 245/40 size tires are actually a "better" fit to the wheels than the 255/40. (Only 0.2% difference from the "true" OEM wheel; which are the 17x7.5 wheels)
Also, they are CHEAPER and more widely available (More manufacturers making this size).
So if you want to save money, get a better "fitment", or want to stick with a certain tire brand (and they don't make 255/40), then go ahead with the 245/40/18 tires.