The current research was focused on the investigation of the effectiveness of a new behaviour-analytic implicit psychological measure, the Function Acquisition Speed Test (FAST) in quantifying differential relatedness between experimental stimuli. Demonstration of the effectiveness of the measure in this regard would provide salient information to implicit measures researchers regarding the specific behavioural processes quantified by these measures, as well as providing stimulus equivalence researchers with a novel, continuous measure of differential relatedness. Experiment 1 (n = 62) investigated the effectiveness of the FAST in quantifying differential stimulus relatedness varied along inter-class parameters (i.e., a differing number of training iterations). Experiment 2 (n = 16) investigated FAST’s efficacy in measuring differential intra-class stimulus relatedness, with subjects completing three FASTs with stimulus pairs of varying nodal distance based on training. The FAST showed effectiveness in both experiments in detecting differences in experimentally-varied relatedness. The findings from the current experiments suggest that the FAST is an effective measure of differential stimulus relatedness, providing an empirical basis for a behaviour-analytic stimulus relatedness account of effects seen in implicit measures. As well as this, the FAST shows promise in quantifying the emergence of stimulus relations during training procedures. Overcoming the conceptual and procedural opacity of other implicit measures, the FAST may be suggested as a functionally-understood, conceptually-coherent implicit psychological measure, with great potential utility in the quantification of stimulus relatedness.