ONLY 17x10+X FN's are concave. None of the others.
18x9.5 FN's are only slightly concave:
I have a set of 17x10+25 FN's and they are concave.
All 17x10 FN's (+12, +25, +35) have the same concavity because they increase the offset from +12 by grinding the hub face down farther.
/thread.