Just about anything added to the outside of your car is going to produce drag and reduce fuel economy. That said, I have the splash gaurds on my Prius and I'm still getting great numbers. I look at it this way. A little lower FE because of the mud guards is keeping me from wasting lots of water washing the black car every other day. Do what you want and find your own rational. A hybrid with a little extra drag is better than an SUV with one occupant. IF we don't enjoy and customize our cars to our liking we are less likely to attract the averge joe to the hybrid market. I pretty much draw the line at the big spoiler wings on the back though!

Prius 06 Black Pack#2