Originally Posted by Nikolas
What you need to find out is the overall diameter of each tire. If you go to TireRack you can get the specs for the tires they carry. They list everything including sidewall, diamter, revs per mile etc. You want the front and rear diameter to be as close as possible - less than 1% difference. Since you are proposing the same profile of 40 and different widths, I am pretty sure that combination would be very bad for XI. I would stick with a 255/40 in the back and not worry about it. The reality is the wider the track, the better traction you will have in most packed snow and ice conditions. The only time that narrower theory of snow tires applies is on fresh untracked snow where there is less plowing resistance with a narrower tire.
Well, Nokain doesn't make a 255 40, and the 245 40's are 1.32% variance on the entire assembly diameter. I'm going to re-order 225s for all corners.
Thanks for the help guys.