Mike,
I have copies of two technical reports,
1. Airflow Beneath an Automobile, SAE Technical paper 741028 by Buckley and Laitone.
2. Performance and Design of Straight, Two Dimensional Diffusers, from the March 1967 Transactions of the ASME by Reneau, Johnston and Kline.
The first is 7 pages, it is interesting to note that one of the sponsors was Collin Chapman's Group Lotus Car Co. The second is 9 pages. Both are sprinkled with formulas alas none of which calculate drag. I copied them both from the Lockheed technical library back when I was contributing to the GNP.
Back in the 80's, with the aid of an engineer from the good old lazy L, I designed and fabricated skirts for our modified roadster which effectively converted our flat belly pan into a ground effects tunnel (diffuser). We already had rake, going from 3" ground clearance at the firewall to 9" at the rear. The short story is that it worked great, we pulled all of the weight out of the car (500+ lbs.) and traction was seemingly unaffected.
Wait for it!
However if the car pivoted even slightly because of wet salt or wind gusts the flow stalled and the car spun instantly.
We went back to ballast.