This study concerns short-cut aramid fiber reinforcement of synthetic elastomer compounds and their influence on the processability and mechanical properties. Short-fiber reinforcement of elastomers is very complex, because it depends on many mutually interacting factors: fiber concentration, fiber orientation distribution, fiber length and distribution, fiber-matrix interfacial strength, and properties of the matrix. The relationship between these influencing factors is highlighted in an S-SBR compound by design of experiments. Two 3 mm long aramid fibers were used: an epoxy-amine–coated fiber and a virgin fiber without coating. To potentially achieve a fiber–matrix interaction, the following silane coupling agents were employed: bis-(triethoxysilylpropyl)-disulfane (TESPD), bis-(triethoxysilylpropyl)-tetrasulfane (TESPT), S-3(triethoxysilylpropyl)-octanethioate (NXT), and an alkylpolyether-mercapto-silane (Si 363), all in combination with the adhesion-activated aramid fibers and in comparison with the virgin fibers. They are compared on equimolar basis with regard to the amount of reactive ethoxy groups versus TESPD, making use of a ‘‘design of experiments’’ approach of the experimental setup. The outcome shows that, contrary to common assumptions, the effect of the fiber–matrix interaction is grossly overshadowed by the effects of other factors (i.e., fiber concentration and orientation) on the vulcanization system. For each mechanical property response, an optimization prediction is calculated and confirmed with an experimental run, showing, for example, a 330% potential improvement in the Young’s modulus.