Thermal design of industrial vehicles needs to estimate temperature of coolant for engines to prevent overheating. Temperature of cooling air into a radiator, however, is unknown if the vehicle has unknown heat sources on the windward of the radiator. It is therefore difficult to estimate the coolant temperature with good accuracy. This study has made some suppositions regarding the unknown heat quantity in excavators to obtain empirical formulae of the cooling air temperature. The formulae have been obtained by regression analysis based on experiments. Finally, the formulae have been validated using another excavator.