it's scary to be looking at new rubber/wheels for the spring, not knowing for
 

sure, as you said, if the same sizes with another wheel/tire manufacturer will give me issues.

how can we get a handle on this?
measure our tires/wheels off the car?

Not sure...all you can do is talk to others with the same et/tire that you're looking at.
 

When I bought my 19's, I'd prior spoken with a guy on Fourtitude who had et45 with 235's, was lowered and said he drove aggresively with no rubbing...so I thought I was fine buying et48 with 235's....didn't work for me. Quite a PITA...Some sites give manufacturers specs on tires that may show if one 225 is wider/narrower than another...haven't really looked into it tho.

FYI I've got PS2's right now..235/40/18. Mine are the Porsche version of the tire, a buddy of mine has them on his S4 in the same size but the regular version. Mine have a WAY bigger rim protector on them so are probably going to have a different sidewall profile etc being as it's from a different mold. Kinda annoying that there seems to be such variances between diferent tires/manufacturers for what is rated as the same size tire.