They're fat-tired open-wheel cars. The high drag comes from the tacked on wings. The underbody does contribute downforce. It does it by expediting the airflow through a closed plenum with tunnels and induced vortexes. I don't think that adds drag, but I've been wrong before, once.
I talked above about a 'pontoons enclosing the wheels'. More specifically look at the spats on the Template as Xist showed at Permalink #16 (that you keep cutting off). And look at basjoos' Aerocivic:
http://ecomodder.com/forum/showthrea...1-0-a-290.html
https://www.aerocivic.com/
It has double-walled side skirts flushed to the inner and outer walls of the tires, 'catamaran-style'. It has zero break-over angle but I think the skirts are conveyor belt material. Notice also the wedged out nose and high boat tail. They work together.
Here's my own theory on how to terminate a Template shape (this one is squircular):
It is an half-bellhorn diffuser that couples a flat bottom to the truncated tail.
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..and should work as well at half the speed, but not at full speed.
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It's all the same from approximately 25mph to approximately 250mph. More benefit going faster.