Pressure drag. It goes up with the square of the velocity.
Before society got a stick up it's b*tt, you could drop a fiberglass 917 splash copy on a Beetle floorpan and get 0.40 to 0.30 drag reduction. Fine for traffic downtown if you can handle the ingress/egress and lack of stowage.
edit: You have five posts now, you can use the Link button:
https://www.yankodesign.com/2017/12/...r-semi-trucks/
My proposal for that has a solid panel which makes the roof of the diffuser.
