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All three sizes you listed will work however the 235/40/18 outer diameter is too small, imo.
Except for F34, all other bmw F-series have 26" diameter tires. 235/40/18 is 25.4". It will lower the car .3", increase fenderwell gap .3", provide less impact protection and a less comfortable ride. It will also spin 17 more revs/mile, thus accruing more mileage than you're actually travelling. granted, theyre small differences but that is the affect of it.
Personally, I'd stay closer to, or a little greater than, OEM diameter, but If that doesn't bother you 235/40/18 works.
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