I was wondering today (and I think I have this right) the idea is to get rid of the drag behind the car, boat tail best, kammback another option. However my car is vertical on the rear
New Kia Picanto in Lakeside and Southend | Essex Auto Group
As neither of the above is an option what about creating a vortex behind the car. Could a strip of this across the back have any effect?
Prikka-Strip intruder and cat excluder | Fence spikes | Anti-Intruder