Here ya go, some numbers to go with the theory and forms.
Aerodynamics Research Revolutionizes Truck Design
https://spinoff.nasa.gov/Spinoff2008/t_3.html
Quote:
Rounding the vertical corners on the front and rear of the cab reduced drag by 40 percent while decreasing internal volume by only 1.3 percent. Likewise, rounding the vertical and horizontal corners cut drag by 54 percent, with a 3-percent loss of internal volume. Closing the gap between the cab and the trailer realized a significant reduction in drag and 20 to 25 percent less fuel consumption. A second group of tests added a faired underbody and a boat tail, the latter feature resulting in drag reduction of about 15 percent. Assuming annual mileage of 100,000 driven by an independent trucker, these drag reductions would translate to fuel savings of as much as 6,829 gallons per year.........................
Perhaps most importantly, drivers of vehicles fitted with Airtabs have reported improved stability and handling and dramatically reduced fishtailing of trailers—an effect where the trailer sways or slides from side to side independent of the tractor, potentially causing catastrophic loss of control—effects that are especially important with the double trailers found in North America and the famous quad-trailer “road trains” in Australia. Increased stability also means that the trailer does not scrub on the sides of the road as much, increasing the life of tires. Drivers also report better handling when being passed in the same direction by other large vehicles.
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There is half-hearted acceptance of Airtabs in the forum in my opinion, speaking for myself, I was curious but cautious about jumping on any bandwagon I didn't completely understand. Maybe I will take a second look now.
Farraider, as far as adding a #9 to the sketch for Airtabs I will leave that up to you.