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Sidewall height is relative to tire width.
I assume (I may be wrong) that your tires are 225/40x18 front and 255/35x18 rear.
In this case, sidewall height is 225mm * 0.40 = 90.00mm for front tires, and 255mm * 0.35 = 89.25mm for rear tires.
Both front and rear rims are 18", so the wheel diameter is about the same front and rear. Keep in mind that there's some acceptable tolerance. For example, the diameter difference between a brand-new tire versus a fully worn one is about 12-15mm.
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