The Exhaust Gas Recirculation (EGR) rate is a critical parameter of turbocharged diesel engines because it determines the trade-off between NOx and particulate matter (PM) emissions. On some heavy duty engines the EGR mass flow is directly measured with a Venturibased sensor and a closed loop control system maintains EGR flow. However, on most light duty diesel engines the EGR mass flow must be estimated. This paper compares two methods for estimating EGR mass flow. The first method, referred to as the Speed Density method, serves as a baseline for comparison and uses sensors for engine speed, intake manifold pressure and temperature, as well as fresh air flow (MAF). The new, second method adds turbo speed to this sensor set, and includes additional engine modelling equations, such as the EGR valve equation and the turbine equation. Special measures are taken to allow the additional equations to execute without issue on production ECMs (Electronics Controls Modules). Sensitivity analysis implies that the new EGR estimation method is less sensitive to error in any given sensor or modelling equation than the baseline EGR estimation algorithm. However, this reduced sensitivity comes at the expense of additional complexity and with the inclusion of additional potential sources of error in the estimator. Results on experimental data show less EGR mass flow variability using the new approach than using the Speed Density approach. This implies that, at least on the engine examined, the additional equations removed more variability than they added.