MOTORTREND stuffs the wake for higher mpg?
 

Frank Marcus shared an interesting ( to me ) tidbit in the May, 2023 edition of the above, page 76.
They have a long-term KIA Carnival van, that Markus has done 700-mile road trips in.
On one trip, a 500-pound capacity, receiver-hitch mounted,24" by 60" REESE Explore ProSeries cargo carrier was added.
Mpg for the 700-mile trip registered 24.7-mpg with, compared to 23.6-mpg without.
Marcus isn't ready to attribute the carrier to the fuel economy difference, but it's information.
As an aside, back-soiling of the van's backlight was significantly reduced during rain events.
Hucho's schematic representations of time-averaged squareback wake flow provide some insight into unobstructed wake turbulence transport of liquid water onto a squareback base.
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If one imagines NASA's, 1977, Cd 0.238, 'Project Shoebox' boat tail slipped 'over' that cargo carrier, it makes for an interesting thought experiment.
Something not lost on the creators of the ' 300-mpg ' AirShip L1 RV travel trailer.;)

I assume he had some cargo on the rack, and it wasn't empty?

I see that as reducing drag, even if no thought of aero was put into it. Even a partial fill of that low pressure zone should be a benefit.

This is why I'm putting on a custom lightweight hitch carrier I made for my Prius. I do want to put on a coroplast bottom and some low sides, but that is about it.

Have you ever seen a car on the highway with a temporary paper license plate? It's always blowing upward. That always gets me thinking.

'some cargo'
 

Yes, an unspecified load, on a 500-lb rated Reese Explore ProSeries 63153 Rambler, with the load stored inside a MODOKIT Cargo Carrier Bag, 26" high, with it's 'floor' covering the 'ventilated' metal mesh.
And yes, I've seen the paper plate phenomena.
If you happen to find yourself behind an aging Ford Crown Victoria, Mercury Grand Marquis, or Lincoln Town Car, you may notice the lower portion of the rear fascia 'quaking' as it's buffeted by wake turbulence, something that fell through the cracks during design.:o