Effectiveness of side skirts?

[aerohead]

The only thing I have "scientific" for side skirts is from the development of the 1982 Pontiac Trans Am.--------- The skirts provided a 4.4% drag reduction.------------ When combined with the under-car wrap-around front airdam,drag was reduced 7%,which would yield a 3.5% improvement in mpg at 55-mph.