Aerodynamics - Trailers?

A major aero drag contributor is the gap between the tow vehicle and the trailer, and while you can't easily seal that and make a huge fairing to close it, you can do a number of things to reduce that problem.

A big contributor to the "parachute" effect are the corners of the trailer that are sticking out on the sides beyond the sides of the car.

With tractor-trailer rigs they found that a major contribution to drag was that the air flowing over the cab hit the bluff front of the trailer and then went down between the trailer and cab and was pumped out the sides. This loss of momentum, creates a lot of drag. This drag was higher than if the air had just stagnaned on a bluff feature on the roof becaus of the losses and additional drag cause by the disrubtion of air on the sides. The fairings over the cab of the truck eliminates this effect and substantially reduces this drag contribution.

Since your car is narrower than the trailer, the two bluff surfaces on the lower front of the trailer that are sticking out cause air to turn inward, lose momentum and then get dumped out over the top of the trailer. You don't want the air to hit a bluff body coming of the back of the car, so not only do you want a radius on the on the bottom front surfaces, but you want to angle the surface in at an angle (like 45 degrees or more) until you are inside of the side line of the body, and then put an additional radius on it there. You could alternatively add a bit of fairing to the body to deflect the flow out to the sides and achieve the same effect.

I also recall seeing some testing that showed a substantial reduction in drag could be obtained by sealing the horizontal gap between the trailer and tow vehicle. This prevented air under the car being sucked up into the gap between the trailer the tow vehicle. This may not be that practical to achieve, but covering the tongue v so that air can't pass up through it, then adding a flexible surface between the back bumper and the trailer would finish the job. The air flowing over the car and trailer are at a higer velocity and lower pressure than the air under the vehicle and it will get sucked up into the gap and then reaccelerated and that creates a lot of drag.

 

Is the extention at the back of the trailer strictly for trailer lights or is it part of the loading ramp?

As others have said, if it is strictly for trailer lights then that certainly becomes your biggest target for reducing air drag. If it is also part of the loading ramp (hinged at the bottom ???) and is not something you can change, then maybe your looking at a fairing at that location.