It doesn't look bad either. My car had 16x7.5 wheels, when I bought it and 225/50 tires. I don't think it looked bad. For the winter I put 205/55 on 'em and it still doesn't look bad

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As I found out, you don't need to install 225 to x8 or 255 to x8.5. That's just preferably how you should do it. For example, you can install 225/40 R 18 tires to wheels, which are 18x7.5-18x9, while 18x8 is preferable. It may look not so good at boundary sizes, but you won't notice +/- 0.5.
tirerack.com has such information at the specifications page of tires.
As for me, when spring hits I'll just get 18x8.5 and install 225/40 all-around. I'd better get 18x8, but the wheels I want to get are not available in that size.
I don't think my 328 or your 320d need something bigger than 225

. 245/255 will be only for looks with no gain in perfomance.
With ****ty roads here I'd better keep unsprung weight at the minimal level.