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The rate of heat rejection to the coolant system of an internal combustion engine depends upon coolant composition, among other factors, because this influences the coolant side heat transfer coefficient. The correlation developed by Taylor and Toong for heat transfer rate has been modified to account for this effect. The modification retains the gas-to-coolant passage thermal resistance implicit in the original correlation. The modified correlation gives predictions in agreement with experimental data. Compared to 100% water, mixtures of 50% ethylene glycol/50% water lower heat rejection rates by typically 5% and up to 25% in the extreme. This depends upon local conditions in the coolant circuit, which can give rise to different heat transfer regimes. Application of the modified correlation is outlined and illustrated.