Well, Skate, since no one with 18s is rushing to reply, let me say that:1. The wheel diameter is not a factor in clearance unless you get really ridiculous. Offset plus tire width and diameter are what matter.2. I have 17" x 10" wheels on my Vibe (lowered ~1-1/4") and they clear fine. The 275/40 tires are tight, but I'm sure you plan something more modest!3. I had these wheels lying around, so I used them. If I was buying new, I wouldn't go beyond 245s, at something close to stock diameter. 4. 245/40 x 17"s equal the stock diameter.5. 235s would likely be necessary with 18" to do the same.6. The specification section at a site such as
http://www.tirerack.com has all the data needed to match stock diameter.7. Research twice, buy once!