A Savonius rotor used as a speed measuring device was subjected to a unidirectional unsteady flow having a mean value U0. A comparison between the rotor rotation rate in this unsteady flow and its rotation rate in a steady flow U0 yielded systematic errors in velocities as inferred from steady-state calibrations. Further dependence of the errors on dimensional quantities suggests that pr esent attempts to model the Savonius rotor's dynamics will have to be expanded.