The Questair Venture provides a good example of an aircraft boat tail shape that could be easily adapted to fit a car. This very fast homebuilt has been called "the flying egg" and has a very short tail, but presumably has attached airflow to manage the speeds that it reaches. It has a 48" wide cockpit, comparable in width to that of a subcompact car.
Here are some line drawings of the Venture. Its tailcone seems to exceed the 10 to one ratio that people have quoted are being the sharpest taper that can retain an attached airflow. This aircraft is one of the fastest prop-driven homebuilts and holds a number of speed records in its class.
Here are some line drawings of the Venture. Its tailcone seems to exceed the 10 to one ratio that people have quoted
My understanding is the "10" refers to max "degrees" (from vehicle direction of travel/air flow direction) that a plane can change inward. And one of the Chalmers reports shows that it's actually more of a range, from 10-20, with a sweet spot in there somewhere.
I also read a Phil Knox quote recently that says an elliptical curve (beginning from parallel to wind flow) to up to 20 degrees inward can still maintain attached flow - you just can't go to 20 degrees instantly. That's my understanding, anyway. Would explain the appearance of this airplane.