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If I were to tow an Airstream trailer with the rounded front, behind a truck with a camper shell, how close to the back of the truck would the trailer have to be in order to benefit the Cd?
Welcome
The big truck manufacturers want to see no more than about 24" between tractor and trailer for FE. On my TV/TT rig the gap is 60". On some nicely done custom hitch rigging jobs (all of these are aluminum aero TT's) the gap can be down to 42". IOW, treat the TV and TT separately for aero. No wing or funky bed topper will benefit the "gap" effectively. The gap has to be shorter.
Conversely, some 5'ers can have a nice short gap. See posts by Kamper Bob and Skyking. The other trade-offs of a 5'er don't make it a worthwhile choice past a certain size, IMO.
As to the rest, you'll want to define the life and the miles expected of the truck. Without that it's pocket pool. Will the truck have IRS deductible miles? Will it earn it's way, IOW? Or is it an expensive commuter vehicle, worthless 4WD and the rest? Because there are some alternate TV's that can pull even a big A/S TT that are also far better when solo for safety, FE and general comfort.
I compared my truck to another man in the same climate/terrain as mine driving an exact replica of mine but with a gas motor with the same miles solo & towing, plus roughly city versus highway. So long as diesel was no more than 50-cents/gl higher than gas then the fuel price difference was irrelevant. But that is only part of the story. At 8-years and 180k miles I am about halfway through the life of my truck. With under $400 in repairs, and a maintenance schedule better than the gasser. Two sets of tires and brakes in the first quarter million miles is how it is looking. Conversely, a gas motor truck is nearing end of engine design life, [B50] at the same point in time.
IOW, initial purchase price can be more than offset by miles/time. MPG is just icing on the cake.
State your definitions of use plus life time/miles more clearly. Miles solo, miles towing, etc.
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