I have seen them on just about every car from a big Audi SUV to Corollas.
They all just look like an inverted Gurney flap to me. Which makes me wonder if they generate lift.
I have been thinking about just how big I could make them without scraping, and I just had one of those thingers where a light bulb turns on.
Contrive some sort of contraption that is mounted to the suspension/steering knuckle that is exactly the size of the wheel!
If made of ABS, I can't imagine it would add too much unsprung weight.