Low Temperature Combustion using compression ignition may provide high efficiency combined with low emissions of oxides of nitrogen and soot. This process is facilitated by fuels with lower cetane number than standard diesel fuel. Mixtures of gasoline and diesel (“dieseline”) may be one way of achieving this, but a practical concern is the flammability of the headspace vapours in the vehicle fuel tank. A mathematical model is presented that predicts the flammability of the headspace vapours in a tank containing mixtures of gasoline and diesel fuel. Fourteen hydrocarbons and ethanol represent the volatile components. Heavier components are treated as non-volatile diluents in the liquid phase. The non-ideality of the blends of hydrocarbons and ethanol is accounted for using activity coefficients. Predictions for DVPE, vapour phase composition and flammability are compared to experimental data for 12 mixtures of 4 base gasolines, some containing alcohol, a single diesel fuel and various quantities of additional ethanol. A 5% fuel tank fill level and a total tank pressure of 1 atmosphere were used. The model predicted DVPE for both base gasolines and dieseline blends that were within 2-4% of measured values. Predicted upper temperature limits of flammability were consistently 5-10°C higher than measured in this apparatus. The discrepancy was attributed mainly to the impact of downward flame propagation in this apparatus, compared to upward propagation used in flammability data found in the literature and used in the model.