In the winter I run goodyear F1 allseason in 225/40/18 on stock rims with no issues (speedo reads about 5 mph slow).
http://www.msprotege.com/forum/showthread.php?t=123740513
Its shouldn't read that. It should be reading 1.2 mph too fast. SO when your doing 61.2mph, your really doing 60MPH.
This site is everyones friend.
http://www.1010tires.com/TireSizeCalculator.asp?action=submit
Running the Kumho Ecsta XS's in 225x40x18 with absolutely NO issues, other than more grip than a monkey in a tree. (on Cobb springs)
Car pulls like a freight train - if you enjoy your wheelspin, don't buy the XS's!
I'm using a 225/45 on the stock wheel no problems at all. Personally, I won't run a 40 series on a 7" rim. Some are maybe they will have some input for you...