Here's a link to NASA's 'Project Shoebox.'
They modified a FORD Econoline van, and reduced the drag coefficient, from around Cd 0.50, to Cd 0.238, with a 'nasty' boat-tail.
Rather than list all they did, I'll just let you experience it for yourself.
PS In the past, truck makers published their Cds. You may be able to find out, roughly, where you're starting from.
https://history.nasa.gov/monograph46.pdf