>What's the biggest summer radial tire size that I can install on a 15"
>rim. The OEM size is 225/60.
1) The diameter of any tire, if it's the same (+/- 4%) will work.
So, you can go to a 55 or 50 or if you're really nits, 45 series tire.
2) The width of the rim will dictate how wide you can go.
You could go wider but it starts getting silly. I wouldn't go more than 235.
Once more with feeling:
http://www.mbz.org/sys/tires

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> What's the biggest summer radial tire size that I can install on a 15"
> rim. The OEM size is 225/60.
>
> Blair...
Why not ring up BMW in Germany and ask for a recommendation?
They will probably suggest 225/60, which I suspect might just be why they
fitted it in first place.