[Nuson]

some one is likely to come along with a better sounding answer, but the pitch diameter is the diameter where proper meshing should be. So when using gears that "mesh", but doesn't have the expected pitch diameter, the true pd is not being achieved in one, the other, or both gears. In that case, it's hard to say what ratio they're running at. Perhaps it's somewhere between the ratios using the dp of the correct gear and the substitute.