On the OEM 17" x 7.5" rims the offset is +30mm, and the backspacing is 5 3/8". [Measured and calculated].
On the OEM 18" x 8.0" rims the offset is +25mm, and the backspacing is 5 1/2". [Calculated by nominal width plus another 1" for the lips, divided by 2 gives a centerline of 4.5". Then add an inch for the + 25mm offset, and the backspacing is 5.5"].