Z4 has 18 x 8 inch wheels front. 18 x 8.5 inch wheels rear

X5 has 18 x 8.5 inch wheels front and rear.

But, tires are different.... X5's got taller tires = meaning same inner diameter, but larger outer diameter....

Again, Z4 tires:
225/45R-18
255/40R-18

X5
255/55R-18