[ttoyoda]

So as an extreme example, when you have a tailgater 'drafting' behind your ultra slick car, he is actually ruining you aerodynamics by following too closely ?
You are pulling him in your wake, and if it is a large SUV then he just ruined your aero big time.

I don't know about that. I would argue that if your tail-gator is close enough, and most any shape, he acts as an extended tail. The flow gets fully filled in around your tail, and the tail-gator gets the separation drag behind his tail (assuming his flow seperates). Don't forget that the tail-gator is providing power to keep his vehicle moving.
If you were to take a teardrop shape and cut it in half, (the front half is the semi-circle and the back half is the triangle section) but keep the two halves together, and measure the drag force on the front half and the rear half, each would contibute to the total drag.
I do notice that if a huge truck tailgates me I can feel the car speed up, but the following distance of the truck is too close for me to be comfortable with.

So if I drive a Hummer and had an Aptera tailgaiting me, then the .Cd on the Dummer would go down as well since the flow would reattach onto the Aptera ??

To some extent yes. It would be best for the hummer if the front end of the aptera was cut off flat, and the aptera needs to follow close.

I have read about this sort of thing - reattaching flow and such.

Read more. This is the key to the drag of most shapes we can drive on the road, at the speeds we can drive them. Once you get to really streamlined shapes like wing profiles, and aircraft speeds, the surface skin drag becomes more significant, because there is no (little) flow separation. As the speed of the aircraft approaches the speed of sound, all the rules change, so do not make your car look like a supersonic fighter. All the calculations and equations for airflow of low speed objects assume the air is not compressible. That is obviously not really true, and is what changes everthing near the speed of sound.

Lastly, bullet shapes are chosen for many reasons, not just drag. Many bullets are supersonic as well. Any bullet that makes a ripping noise as it flies thru the air is going sonic or supersonic.