The mythbusters car was a later model Ford Taurus 2001 or so. the one with the blunt rear as opposed to the sharper smother rear of the earlier models. They put a layer of clay on it, tested the millage with the clay, then put dimples that were about 3 or 4 inches round. I suspect the dimples made the air flow round the blunt rear which improved the millage. As I recall the fuel milage smooth was worse than my former 1999 Taurus that I had, and about the same or slightly worse with the dimples.