I like to kick them on about 10 to 15C above the engines coolant thermostat's opening temp and turn them off at the thermostat open temp. You usually find the more heat tolerant an engine is, the higher the temperature of the standard coolant thermostat. When the thermostat opens, this is where the manufacturers consider the engine to be at the start of normal operating temp. Normal driving, I also switch my fans off over 2500rpm so they don't come on when I need my power. Works ok because anything over 50 to 60kph I have the airflow through the radiator to not need the fans. Vss is the better second parameter but I don't have it. Keep in mind if you like doing burnouts, you don't want to turn off the fans and I have a manual override for for those occasions.
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