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The case is, that i bought a new set of wheels OZ Superleggera 18x8, but i assume, that tires in original size 255/40/18 won't fit on them.
So what is the next best option?

I currently run 225/45/18 on stock 18x7.5 winter wheels for winter and last summer i did run the original 255/35/19 on stock 19x9.5 "five spoke", but the roads here are so bad, that for the comfort i want 18" for the summer also.

Where is 'here'?

255/40/18 should give you exactly the same rolling diameter as 255/35/19 which is the correct size for the OEM 19" wheels (which are 9" wide, not 9.5"). Question is, would the 255s sit properly on a rim that's only 8" wide. Not sure. It might. Worth a try.

If you run 225/45/18 then that will also be very close to the correct rolling diameter.

By "here" i mean Estonia, sry for that : )

Yes, i know, that the diameter of stock 255/35/19 and 255/40/18 is excactly the same - 661mm and difference between stock winter size 225/45/18 is only 1mm (660mm).

Now, the next option for 18" inch would be 235/45/18 with overall diameter of 669mm, thats only 8mm more - that should do, but the problem is, that it is a uncommon size.
More commonly used is 245/45/18. Sure it will be comfortable, but i don't know will it fit.

235/40/18 would be a more common size still - and even more ideal width wise on a 8 inch rim.