it seems that in my case the added frontal area caused more drag than it cured. I would use it as a case by case thing. It all depends on the car and it's particular underbody aerodynamics. Some cars once a full belly pan is installed no longer need an airdam.
Today my TANK average has crept past 37mpg I'm still baffled I mean I have my 5 spoke heavy 16" wheels with performance tires on the car the lower grille block is off and I'm still climbing. to quote Alice "This just gets curiouser and curiouser."
the wheels are 16X7 and weigh in at 35# each Plus tire it runs close to 50# a corner the tires are 215/55r16 Pirelli P6 and they are nearing shot. I am trying to save up for new springs so I can raise the car back up a little and get the alignment closer to Zero camber.