Size of spacers?

I recently purchased some Miro 318 rims 18x8 with 38mm offset for the new 2007 328xi 4dr. The tire size that is recommended is 225/40/18. After they are being put on, I notice that rear spacers are definitely in need. So, my question here is to see what size of spacers would make the wheels flush with the fender? I am guessing 25mm is good for the rear, but will I need anything like 5mm or 10mm for the front? Please advise!!

With your tire spec. IMO, you can go with even 15mm in the front since the front fenders are flared out much more. 25mm..hmm..you might want to do some research on that. Its a big diff.

Be careful using spacers as using them gets mixed reviews in these forums. They put added pressure on you suspension and wheel bearings. You would be better off getting a set of wheels with a more agressive offset. I did a lot of research on spacers when I was trying to make a set of wheels I had on my E46 fit my new E92. I ultimately decided to get new wheels. If you do decide to get them, I believe you can only put a minimum 10mm spacer on these cars as anything smaller would be susceptible to cracking. Someone please correct me if I am wrong.