The flowers of Hoya lithophytica are virtually identical to the flowers of H. wightii ssp palniensis except for points on the coronal lobes, but that is where the similarities end.  The blooms of H. wightii have a powerful intoxicating scent; the blooms of Hoya lithophytica have no detectable smell.  Hoya lithophytica is a small extremely slow grower, and H. wightii grows rampantly and gets very large.  Below are the flowers of Both Hoyas.

Hoya lithophytica

Hoya wightii