Quoting EHmotorsports:
good info. I love those kind of articles. there is a reason any performance based car has under car trays or panels. same goes for fuel efficient cars.
I imitated this on my old Mini. Flat underbody alone netted me almost +2mpg on average. Another guy that had a 1" drop that got his RLCAs out of the airstream and a homemade diffuser saw more than that for very little investment. Homemade and tested ftw.