Re: (dneal)
I think the reason 225-40/18 isn't recommended is because most of those tires need an 8" rim (the Volvo 18's are 7.5"). But check the brand specs - 7.5" is usually acceptable for these tire sizes.
Also, that's not a lot of sidewall. The ride will be pretty harsh, imo. Of course, this is from someone who thinks the 17's ride a little hard.
Tom.