However, I can't see any way in which the response time of the thermostat can explain how for example when driving at a steady speed on a fast road the temperature can run steadily well below thermostat temperature. Quick brain dump to stimulate yours, based more on physics than knowledge of car engines, nothing excluded, common sense set to OFF: The coolant is being cooled somewhere other than the radiator. The thermostat doesn't stop all flow through the radiator The temperature is being measured at an inappropriate place for comparison with the thermostat rating.Jonathan