I answer a lot of questions on Quora. Many of those questions are about automotive things that may be of interest to CarNewsCafe readers. Like this one.
My Answer: Depends on the setup, the situation, and the deployment. Most of those are worth anywhere from 3–10 percent efficiency gain via aerodynamics. Side skirts for the trailer, for example, can be 2–5 percent on some trailers, less on others. Adding things under the trailer doesn’t necessarily add onto the effect of skirts, but it can bolster their overall effect.
Trailer tails can mean about 5 percent efficiency (fuel economy) gain on most trucks. Most truckers who’re using trailer tails also point out that there is generally less trailer sway and drag, which improves safety as well. This is because of the way the tail skirts change airflow behind the trailer, reducing the vacuum space that can cause a trailer’s rear end to sway when airflow is disturbed (a side breeze, another truck alongside, etc).
A top kit on a trailer can also reduce aerodynamic drag and improve efficiency by 2–5 percent. These often work in tandem with trailer tails.
Other improvements can be made to the tractor (truck) pulling the trailer. These usually aim towards pushing the air that’s underneath the truck up and out before it reaches the trailer and counteracts the help that skirts and axle covers might be doing.
The North American Council for Freight Efficiency has been conducting real-world studies of trailer skirts and other common aerodynamic kits for about a decade.