Aero undertray, wouldnt make a rats **** of difference around at slow speeds. if you were using it at 80kph speeds for extended periods of time, say commuting from gold coast to brisbane or geelong to melb etc, it would clean up the airflow under the car, and there would be neater airflow out the back, which would reduce drag, and therefore increase economy. realistically, unless you had a full undertray, your benefits would be VERY minimal maybe 2-4%. With a full undertray, and a rear spoiler designed for aero benefit, and not for looks, you could potentially save 10%.
<pops 2c into the tin>