alright so i got my 18's test fitted on my tC at the body shop.
18xs7.5 40or45 offset
225/40/ZR18
and there doesnt seem to be any interference issues.

now my delema here is that there isnt much of the wheel stud for the nut to engage. what is the rule of thumb for how many threads is safe for proper engagement. the nut is a M12x1.5 and tire rack says 6.5 turns. ive also found info where they say it should engage minimally the amount of the diameter of the stud.

any other opinions?

I would say at minimum 6 threads, 8 or more being ideal......That is what we use to gauge at discount tire.

How much is not much?

at least diameter length, thats what I read from past post here.

At least 6 full threads, or rotations of the lug nut, is the minimum, for properly torqued lug nuts.

ditto on 6+ turns

I can confirm 6.5 turns minimum for 12 X 1.5mm!