I have found a few threads here about projects for making underbody diffusers. Here is a dissertation I found that may be of interest to some ecomodders:
"An experimental study of automotive underbody diffusers", by Lydia Jowsey, 2013. It was a bluff-body wind tunnel study that included variables of diffuser angle, number of channels and ride height. The material is quite interesting. Link:
https://dspace.lboro.ac.uk/dspace-js...013-Jowsey.pdf
At the end are appendices with data tables from full scale tests done in 2009 of a Ford Focus and a Ford Modeo hatchback. Unfortunately, the Focus model was not mentioned (sedan?, hatchback?), and these tables are not discussed anywhere in the document. Cd is provided and is of interest, but much that is in the tables is cryptic. Maybe someone else can make heads or tails out of them, particularly what led to the minimum Cd figures.