According to one exemplary embodiment, a virtual turbine speed sensor for a multi-stage turbocharger system of an internal combustion engine is disclosed. The multi-stage turbocharger system includes at least two sequential turbochargers each having a compressor and a turbine. The virtual turbine speed sensor includes a compressor efficiency module, an inter-stage air temperature module, and a turbine speed module. The compressor efficiency module is configured to estimate an efficiency of a compressor of a first of the at least two turbochargers. The inter-stage air temperature module is configured to estimate an inter-stage temperature of air between the at least two compressors. The inter-stage temperature estimate is based at least partially on the efficiency of the compressor of the first turbocharger. The turbine speed module is configured to estimate a speed of a turbine of a second of the at least two turbochargers. The speed estimate of the turbine of the second turbocharger is based at least partially on the temperature estimate of air between the at least two compressors.