The front spoiler was effective and did a lot of work but as Brian stated, the rear spoiler only became effective at high speeds. On normal cars and street driving the spoiler is really there for good looks. Almost every Mustang is different in wheels. color, stance etc. and so the optimum spooler angle will differ slightly to give it the best look. As long as you don't go for some radical angle, I would aim for maximum sexiness. Councours cars can be sexy as well!