Someone created a thread saying they did the math and they calculated that even though they would wear out their tires faster, the improved fuel economy would more than compensate.
Apparently it wasn't this thread:
I did the math..
In that thread Dave Lobi said that he made an air dam and grill block from lawn edging and if it increased his fuel economy by 0.5 MPG it would pay for itself over 20,000 miles if it cost $20.