I have 245/40 Nokian Hakka R's on OEM 18s and they work great. they are pretty stretched in the back but no issues yet. They are also significantly shorter than the normal 265/40 rear so the speedo error is pretty obvious. If you can, get 4 OEM 18 front wheels and add some spacers to the rear for a perfect setup. If you don't want to do that, you'll be fine.