Khumo 215/35 18 x 9 see below (the wall pulls back just enough to tuck the tread under the arch whilst leaving only a tiny amount of the wheel lip showing)
below is a 225 40 18 falken fitted to a 18 x 8.5 - it looks as it has provided more stretch - maybe as the wheel is white and or because brands will vary (i have been told that the size you buy has 10% worth of tolerance) also (not teaching anyone to suck eggs) it looks a little different as the side wall is different 225 = the width 40 = the % of wall to the width (not the size in mm as i once thought)
between the two pics one in fact has approx 1cm more of sidewall
225 40 will go on just fine but not much stretch as it is perfectly fine to fit to a 9 inch rim in the first place. the tyre will angle back a little though.
explanation:
according to the calculator a 225 40 17 will fit on any wheel size between 7.5 x 17 - 9 x17 if you want the more stretched look try one size down
215 40 17 which is recommended for wheel sizes of 7x17 - 8.5 x17
this would be the one tyre size down from the recommended tyre size...would give the same look as a above but provide a little more sidewall and be easier to fit.
again you could afford to go one tyre size down even further but then issues with tyre fitters become a problem, as many wont have a bead breaker (high pressure inflater used to fit very rigid lorry tyres etc.)
go onto
http://www.carbibles.com/tyre_bible_pg2.html
type tyre sizes into (about halfway down)
'An ideal rim-width calculator'
this will give the above info i have written.
BRAND is also very important choose something with a stiff sidewall for stretching khumo, fedral, toyo, yoko and falken are all very good for stretching with khumo being on of the easiest to do, although not hard when only one tyre size down
if its just for drifting then nankang (many crazy stretch videos on youtube using this brand) stretch really well but maybe not something great for wet roads.
i did a little write up om stretching here
http://forum.s15oc.com/showthread.php?t=2568&page=2
i am in know way an expert but i did a little reading when trailing tyres on my wheels when i first put on the firbe glass arches (very nervous on catching them)..... the tyre people i know were awesome and test fitted 4 different tyres sizes for me. if you get stuck on any queries that you think i can help on. pm me and ill call you or msn pm etc