A method for determining charge air mass flow rate comprises a turbocharger speed sensor with a closed loop observer that is based on a modeled turbocharger revolution rate. An open loop charge air mass flow rate is determined in a characteristic field stored in an engine controller and is corrected by the scaled difference between the measured and modeled turbocharger revolution rates. The corrected closed loop charge air mass flow is fed back to model the subsequent turbocharger revolution rate and to update the open loop charge air mass flow rate in the characteristic field.