From Sept 2011 to Dec, 2013, the SPY outperformed the Gold Miners significantly.
However, since Dec, 2013, let's notice that their positions have flipped 180 degrees: The Miners have outperformed the overall benchmark index ETF SPDR S&P 500 (SPY).
Let's notice also that my momentum gauge of the SPY/GDX ratio peaked in July 2013.
That means that for 5 months the SPY outperformed the GDX within a developing and very significant non-confirmation of SPY strength relative to the Market Vectors Gold Miners ETF (GDX).
That momentum divergence is not one of the factors that is driving the relationship in the opposite direction.
Judging from the current juxtaposition of momentum and the SPY/GDX ratio, my work is telling me to expect the SPY/GDX to continue lower towards a 60.00-55.00 target zone.