Standard offset is +45mm. You say -12 offset but what measurement is that ? A site I am looking at says that adding a 2" spacer (when will those yanks change to real measurements of metric) will give you a -6mm offset. Another site shows you can use 255/55R18 with 8Jx18 ET45 rims. Generally for that size tyre clearance at the rear will probably be okay. Front clearance is a different story.
I have 235/60R18 plus an Ironman 2" (actually 40mm I believe) lift. The issue is how close will the tyres be on either lock. There is not a huge gap between the top edge of the tyre and the bottom edge of the strut. If I remember tomorrow I will photograph the tyre in both lock positions to see how much clearance there actually is. There will be a bit less with 255's. Since I have new 235's I haven't been following the threads on here re tyre widths but from vague memory 255s will often fit but you may have problems with guards and mud flaps and they may require a little trimming.
255/55 are within 4mm circumference of 235/60 (2322 v 2318 mm) so it is only actual width that creates a problem. I suppose whether you can use your rims boils down to what a -12 offset is. Your rims at 9" seem to be the equiv of 9J width.
I see 2013GV came in while I was typing. My vehicle is a 3.2l. 255 would fit comfortably at the rear. I'll see tomorrow how the front would go but there is a lot less clearance there.
I am comfortable with both also having grown up with imperial and then moving to metric. I am happy with either. When I mention imperial units to youngsters I get a blank stare.
Onto wheel clearance. I set the car up and positioned the wheels checking for clearance at both the front and rear of the front wheel-arches. Then checking from the outside edges at an angle and adding 2.25" or 57mm plus adding a bit for the wider tyres (from mine which are 235s) there ain't no way in hell you will fit those rims and tyres. Remember I also have Ironmans raised height. I didn't even bother jacking up a wheel to see what the vertical clearance might be like. I did take some photos but it was so obvious they weren't going to fit that I am not even bothering to show them here. That is actually a little bit of a lie because they would fit but you would have to cut away quite a bit at the front and rear to accomodate them. It would look ugly and messy and since the reason for wanting to make the vehicle 'look' better it would kinda defeat the purpose.
I think that moving to 245's would probably be okay and maybe even 255's as long as you had the lift and kept the offset same as factory. Even then 255's might be stretching it as the top inside edge of the tyre isn't far from Ironmans spring mount on the strut. I might feel happier with 255's if you used rims with maybe a +35 offset just to give that little extra safety at the strut. 255's with -12 offset no way unless you didn't want to turn corners.
I thank you all for your advice, the 255 will probably work if I stuck with the stock offset. I do agree on that as I saw pics of one on this site and they stated no issues with the 255/55/18 size. I just happen to have a set wheels that were a bit expensive laying around and was just wondering about them. To be honest I have been too busy trying to figure out why its dying at idle still to deal with the wheel thing right now. I own a brick on wheels and its driving me crazy.....but I do thank you all for your support in my wheel offset question...