The paper discusses the speed estimation accuracy of two Sliding Mode Model Reference Adaptive System observers under improper rotor time constant. The SM MRAS observers are used for speed estimation in a sensorless direct field oriented (DFO) induction machine (IM) drive. In a sensorless DFO drive, the rotor fluxes are estimated based on the measured voltages/currents and are used for fleld orientation. Speed estimation by the rotor flux MRAS method is attractive since the reference quantities (rotor fluxes) are already available. Classic MRAS or SM MRAS speed estimators are viable alternatives and they work well under ideal conditions. The paper analyzes the speed estimation error of the SM MRAS observers under real conditions (improper rotor time constant and nonideal integration) and compares it with the Classic MRAS method.