The fronts will work but a 235/35 would fill out the rim better and out back on a 9.5 rim I think a 265/30 would be a real good fit. I think 255/30 will probably work but you may have a stretched look. Thats when the tire is narrower than the rim and the rim sticks out.
volk re30 18x8.5 / tc kline coilovers / cobb ap / afe stg 2 sealed cai / afe air scoops / 5" ets fmic (sold car)
2013 x3 35i fully loaded with M package