Mixing elements are used for the material and thermal homogenization of polymer melts. A special category are free-rotating mixers like the well-known Twente-Mixing-Ring. When designing free-rotating mixing sleeves/rings, the self-adjusting sleeve speed is not known, which limits the value of CFD-based investigations of mixing geometries regarding mixing and process performance. In this paper a concept is presented to estimate the sleeve speed of a free-rotating, honeycomb-shaped mixing element depending on the polymer material. Based on a numerically calculated flow field the braking and driving torques acting on the sleeve were determined. The rotation speed of the sleeve was calculated for the steady state balance between the braking and driving torques. A comparison between sleeve speeds determined in simulations and in experiments revealed that the accuracy of the simulations results strongly depend on the polymer material.