[aerohead]

Since the debut of the Cybertruck,and the TFLcars 'Gauntlet',Tesla/trailer towing tests I've been running numbers on trailers.
I did a twenty-hour proof,based on the 1969 Dodge Charger Daytona,in a 2-car draft,using the original research by Chrysler Corp.,plus a road test on the car by CAR and DRIVER,conducted at the same proving grounds where the Daytona was originally developed.
Starting with the NASCAR,two-car draft,step-by-step,I reconfigured the Charger trailing car into a pop-up RV trailer.
With a roof-delete,cooling system delete,mirror-delete,addition of a full belly pan,with diffuser,full wheel skirts,underbody 'hour-glass' delete,'features drag delete,Hoerner' streamlined railroad tail car boat-tail,and articulated,'accordion',full gap-seal,the trailer came in at Cd-negative 0.0835,giving the 'rig' a Cd 0.2315,@ 21.8 sq-ft frontal area,with the trailer at Cd 0.01597,15.60 sq-ft frontal area,and 4,251-lbs.
I reverse-engineered a coefficient of rolling resistance for the tires,at Crr 0.01672813,from the CAR and DRIVER road test at Chelsea,Michigan.
At this configuration,pulling the trailer,while adding 4251-pounds,improves the mpg @ 70-mph,from 19.884,to 19.9972 mpg.
This is with the original Good-Year bias-ply tires.If radials were substituted,we'd see a minor mpg increase.
It's an interesting thought experiment,essentially doubling the weight of the vehicle,adding cargo space,while improving fuel economy.The technology dates to 1922.
I'll be running the numbers specific to BZP's Dark Aero.The preliminary data look pretty tasty.This coming October I hope to verify this concept at A2.