go with whatever wheel weighs less(stock one prolly) I thought the stock 240 16" rims were 6.5" wide....maybe im wrong, I got it in my own FAQ for chrissakes.... refer to that :p
an 18x7.5 would be perfect for a 215-40-18 or 215-35-18, the 225-40-18 will be a bit wider, buy may be stretching to fit the wheel...its more than safe though.
your stock wheels can use a 225-50-16, as I have put that fitment on many peoples cars and it works great, and has a nice contract patch, as well as lots of selectin of high performance tires...
hope that helps.also, the esction width will be the same, but the tran on the ground may be greater with the 225-40-18 simply because the sction width will be closer to the tread width via the reduced sidewall...-chetEDIT: holly type-o batman! please excuse all typos.
