The width and size is always stamped in the back of all Oz alloys. If you have measured the width you may have got 8" but infact it is 7"
If you also measure the actual wheel size it will measure 19" when infact the wheel is only 18"...The reason for this is that a wheel measurement is taken from where the tyre fits on the beads. If your wheels are 8" wide it will be stamped 8x18. You may be right and I maybe wrong?