The real beauty of the electric fan is that it is divorced from engine speed. This means that you can have a slow, burbling idle while stuck in traffic and still move plenty of air. This also means the fan can stay off when you are headed down the road and don't need the fan to move air through the radiator. Since (hopefully) you are moving down the road most of the time, you save gas and have more power available most of the time with an electric fan.