I am running a 265x35x22 with a +15 and it will still rub if i take a dip too fast. the +18mm is only .11" closer to the chassis, which couldn't make a HUGE difference... maybe a tiny bit tho. running the 255s instead of the 265 will shave off about .43" which will put you right about .5" (more or less)...
i guess what I am saying is... Cueball is right, no matter what you do, if you hit a dip fast enough, you can still hit the fender.
Roll the fenders! =0) you can rent the tool form some of the forum guys...
hope that helped you a little bit...