You have to concider the recommended spacers too, and they actually decreases the offset of the bare rim ( as I understand it)
So:
For a 645i -
9.0J X 20.0" ET 15 Front with 7mm spacer = 8mm total offset
10.0J X 20.0" ET 13 Rear with 20mm spacer = -7mm total offset

For an X5 -
9.0J X 20.0" ET 35 Front with 7mm spacer = 28mm total offset
10.0J X 20.0" ET 23 Rear with 10mm spacer = 13mm total offset

If i skip the recommended 645 spacers i end up with:
Front 15mm offset (compared to rec. X5 total offset of 28)
Rear 13mm offset (compared to rec. X5 total offset of13)

So, rear is perfect, front will protrude 13 mm more than recommended.
Right ?
Or am I missing something??
Will 13 mm be to "fat" in front?
Or will it look strange compared to the rear?

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2002 X5 4.4i
1959 Ford Skyliner