According to this calculator for percentage difference between stock and aftermarket sizes in terms of combined wheel/tire overall diameter:
http://www.e90post.com/forums/forumdisplay.php?f=12
Your rear wheel/tire setup will be 1.195% off from stock. Which means your speedo will read 61 mph when it's actually 60mph. Negligible difference
BTW, the comparison isn't between your front and rear wheel/tire overall diameter. It's comparison between each corner's individual totals.
i.e., what I did above to get you that result was compare stock rear wheel spec: 18x9 +35 w/ 255/40/18's vs. the aftermarket set you're looking to get: 18x9.5 +35 w/ 265/40/18's.