[BHarvey]

It's easy to see how a boattail works, it closes down the size of the hole you just made in the air, and does it with minimal detachment.

Say the rear of a car from bottom of bumper to top of trunk measures 2.5' and the width from side to side is 4.5'.

Which is more important to close down, the vertical or horizontal?

I am getting ready to start on my first version that consists of a trailer hitch exiting the middle of the rear bumper instead of hanging in the breeze below the car. My idea is to come straight off the trunk deck level with the trunk to about 4' off the car, then have the bottom portion keep the same angle as the rear bumper and then have the sides taper in as well.

All will be in an effort to reduce the departing air to half the size of the rear of the car, or less.

Am going to test before and after for flow over the rear of the car.