For your reference, the stock X5 setup for an 18" rim would be either an 8.5x18 with an ET 48 offset, or an 8x18 with an ET 40 offset. Tire size for both would be 255/55/R18

The rims in your description look to be the Style 93 from a 7 series, which are 8x18 with an offset of ET 24. So there is an offset difference of 16, which is not insignificant. Personally I wouldn't do it.

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