i think your plastic car bling; is expensive, and has no effect; about as good as your defensive tactics are.
considering i'm driving with something similar on my vehicle right now; i find that they are a detriment to my fuel economy.
i went from 400km/tank before to 360-340km/tank after.(if you respond with the comment of it's my fault they don't work as intended, you really are someone with spin control in mind). while i will say they do increase my coast length/speed; i bet if i drove like a "Normal stop and go" person it would be way worse(considering my gf drives it every so often and can go through a 1/4 of a tank in half the distance or less what i would)
oh, and what i have on are:
Aerodynamic Fuel Economy Savers for Road Vehicles