IS_250

iTrader: (2)

Okay i have finally narrowed down a couple wheel choices. Ive been doing the math and i think it will fit, but i need to understand the correlation of wheel width to the offset. I found a set of wheels but the offset is +40 front and rear. 8.5" front and 10" rear. So far the widths i have seen on the 06 IS are

F 8.5" +44 R 9.5" +37 using 235/30/20 and 275/30/20
F 8.5" +38 R 10" +45 using 245/35/19 and 275/35/19
F 8.0" +43 R 10.5" +43 using 245/35/19 and 275/30/19

now the last example was lowered and used a wider wheel but higher offset and did not have any issues with rubbing. What im wondering is if I use a wheel with +40 offset but 0.5" narrower will it fit? As you can see im only concerned with the rear fitment. Im pretty sure that the front will not be an issue. im planning to use 235/23/20 F and 275/30/20 R. For some reason, i think im looking at this way too hard. 0.5" = 12.7 mm so therefore the difference of 3mm in offset should be okay since the wheel is 0.5" narrower right?

SWSC400

see my post in this forum called "Handy offset calculator" I think that might give you the answer your looking for.

IS_250

iTrader: (2)

yeah, i used it earlier as well. its a good tool, glad you posted it. when i used it this is what i came up with for the rear. not sure if it helped me or confused me more.

Stock 8.5" +50 vs 10.5" +43
inner clearance 18mm less
outer position extended 32mm

Stock 8.5" +50 vs 10" +40
inner clearance 9mm less
outer position extended 29mm

so what i gathered is that it wont stick out as far, but i will have less clearance to work with near the suspension. i guess what im really needing to know is if the 9mm difference will be a problem.

SWSC400

It sounds from all the examples you have given that you should be okay, 3mm isn't a whole lot. The calculator says you will have 9mm less clearance, but that dosen't mean it actually won't fit. Like I said, from the other sizes you listed, sounds okay to me. You can always contact the wheel maunfacturer if in doubt which is what I do. Hope this helps!