Mercapto–silanes enable an improved silica dispersion state compared with sulfide–silanes in tire tread compounds, which causes a better balance of wet grip and rolling resistance. However, it also causes a higher Mooney viscosity and a shorter scorch time, which results in processing difficulties. The reason for this behavior is not clear. Because the already deeply investigated silica–silane reaction cannot explain the observed effects, the silane–polymer reaction was considered to be responsible and therefore has been evaluated in depth. The reaction between mercapto–silane or sulfide–silane and an olefin instead of a rubber was investigated in terms of reaction possibilities, mechanism, and kinetic aspects by varying the structure of the olefin. The results reveal that mercapto–silanes and sulfide–silanes affect olefins in different ways depending on the structure of the olefin. These results were compared with those in actual tire tread compounds in terms of the in-rubber properties such as the Payne effect and Mooney viscosity. It was confirmed that the different sulfur functions of the silanes interact in a different way with the polymer, which has a significant influence on the processability, silica dispersion state, and also rolling resistance indicator of the final rubber compound.