Personally, I think unless you're able to or dare take the risks
of driving above 200Kmh, that rear difuser is just there for
looks.
Besides, Rally cars and or sedan cars are bulky enough for weight,
the additional downforce would only increase fuel consumption at
slow speeds, as well as high speeds.
F1 cars are super light, hence the need for winds on both front and
rear ends. Otherwise they are very apt to flip upwards at high
speeds.
F1 cars can travel upside down in a tunnel because of these front
and rear wings. But that's if it is driven up to 300kmh and
above.
