In this paper, we present a two-zone thermodynamic model that allows us to predict the time dependent in-cylinder conditions (P, T) in a high-speed medium BMEP engine fueled with different gas compositions. The details of this model rely on the observation that the measured combustion phasing correlates strongly with the (computed) laminar burning velocity under the conditions existing in the cylinder. To account for turbulence effects, a model parameter is introduced in the burning rate model. Calculations show that for Dutch Natural Gas (DNG)/ethane/propane mixtures a single model parameter, independent of the gas composition, is sufficient to predict the pressure profiles accurately. In contrast, the model parameter for DNG/H2 mixtures shows a dependence on the hydrogen content in the fuel. Adjustment of the model parameter resulted in successful prediction of the effect of hydrogen on the combustion phasing.The accuracy of the pressure profiles based on the phasing model is assessed by comparing the computed autoignition delay times in the end gas based on the predicted pressure profiles with those derived from experimental profiles. Excellent agreement is found, giving us confidence in the predictive power of the two zone model. Using the derived delay times as a measure of knock propensity, the comparison of the predicted propensity with measured Knock Limited Spark Timing (KLST) data showed an excellent correlation. From this we conclude that the methodology developed accurately predicts the knock propensity of fuel gases in the lean-burn, high-speed gas engine used in this study, for all gases tested.