An algorithm for determining the four tuning parameters in a double-Wiebe description of the combustion process in spark-assisted compression ignition engines is presented where the novelty is that the tuning problem is posed as a weighted linear least-squares problem. The approach is applied and shown to describe well an extensive data set from a light-duty gasoline engine for various engine speeds and loads. Correlations are suggested for the four parameters based on the results, which illustrates how the double-Wiebe approach can also be utilized in a predictive simulation. The effectiveness of the methodology is quantified by the accuracy for describing and predicting the heat release rate and predicting the cylinder pressure. The root-mean square errors between the measured and predicted cylinder pressures are 1bar or less, which corresponds to 2% or less of the peak cylinder pressure.