[aerohead]

Some gross figures from Wolf Heinrich Hucho's 2nd-Edition, Aerodynamics of Road Vehicles, NASA,............
* tow-bar length dependent (gap) alone, can mean up to a 8.67% delta Cd.
* a roof fairing, which brings the tow vehicle roofline closer to that of the trailer amplifies the smaller gap modification for up to a 13.6% drag reduction.
* closing off the gap between the tow vehicle and trailer brings drag reduction up to 27.1%.
* reducing the height of the rear separation point by half brings the entire ensemble to a 63.5% drag reduction.
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Lightweight, compound-curve, pre-preg epoxy carbon fiber fairings, gap-filler, and boat-tailing costs very little in weight penalty. So rolling resistance need not suffer significantly.
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If you want to go 'nuts,' you can lower the drag into negative territory, where pulling the trailer actually lowers 'total drag', and you actually increase range when towing. We know this from NASCAR.
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These are just talking points. You're the 'Captain.'