This is how I did it when I bought tires/wheels for my Z.
245 millimeters = 9.64566929 in <-- It will fit but the wheel will stick out a tad bit farther then the tire.
275 millimeters = 10.8267717 in <-- This is what I'm running on a 10.5" wheel and it looks perfect.
You can kind of see how it looks but its hard to tell. The actual wheel is a little bit further out then the tire. Its much easier to curb rash the wheel.
IMG_0487 by
carlili4190, on Flickr
I would honestly go with a 255 on a 9.5"