I did find examples of guys with 235/40s, but on 7.5" rims, not 7".
I ended up going to 225/45R18, for various reasons, and I'm happy with the result.
Based on the clearance I have left, I'd be confident 235s would (just) clear everything, if you have the billies (higher front spring seat). However, from memory 225/45 is considered the MAX width for the OEM 18x7 rims, so I think you'd need at least 7.5, maybe 8" and be very careful with offset.
Hypothetically, 235/
50 would squeeze on the 7" (slopping around corners, and rubbing the spring seat/outer guard ?), but not 235/
40I actually WANTED higher rolling diameter (to correct some of the factory speedo error) and WANTED the tyre slightly wider than the rim (the opposite to the current fad of stretching, which is generating enormous extra business for wheel makers/sellers/repairers). I can now
just rub the curb at crawling speed while parking, and NOT gutter-rash my rim

I took oztoma's advice (sort of) and found these useful.
http://www.carbibles.com/tyre_bible_pg4.htmlhttp://www.1010tires.com/tiresizecalcul ... ion=submitHope my experience/opinion helps

. PM me if you want the long(er) explanation.