[ecomodded]

My understanding is that air under your car is alright its the air dragging against your axles/differentials, shocks and the such that is the real mpg offender.
The under tray hides what it can and the front spoiler/ small air dam keeps air from being too high up in your undercarriage. So make the front spoiler just long enough to hide your differential if you have one, or just long enough to be ever so slightly lower then your suspension/ crap hanging off the bottom of the car.
My opinion is to have the under tray 1/8th to 1/2" above the front spoiler/small air dam.. meaning you do not need a easily damaged extra low hanging front spoiler imo.