Aero drag reduction potential
 

I ran some numbers which can be attached to the Streamlining Template.
I've taken the Cd series from the fuselage data,ran the calcs starting with the Cd 0.04 L/D = 2.5 streamline body rather than the 12 % thickness fuselage of Cd 0.066.
On top of these values I've added 0.04 for the wheels,0.011 for a Korff/Morelli' cooling system,0.007 for 'Features' drag ( cut lines,door handles,wipers,mirrors,trim,etc.) based on the GM PNGV/Precept.
From this,I've 'subtracted' 0.015 for an under-car, full wheel fairing kit,as existed on the original GM Sunraycer or you'll see on the HONDA Dream-2 Solar racer.And I've subtracted an additional 0.008 for basjoos'-style full-floating front wheel skirts.
Since the Template is already broken down into ten segments,one can just pencil in the values below each 10 % segment line beginning at zero.
These values represent a conservative estimate of what we might achieve based on Template streamlining.Since the values are from full-scale wind tunnel testing with critical Reynolds number criteria applied,we should be able to use them with a high degree of confidence.
You can just choose a drag coefficient goal,find it on the Template,and build accordingly.You should be able to predict the new performance before you begin construction.
For values falling between 10 % stations you can just interpolate for a good approximation.

zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115

Note: Everyone will notice,that with proper ground clearance,the under-body of a vehicle projected rearward,would intersect the Template line around 80 %.You'll also notice that there is little drag reduction to be gained by going beyond 80 %.This is the region of 'phantom' tail.
For those contemplating a 100 %- length trailer,you can look at Solaraycer,1957 MG EX 181,Porsche 917 LH,or D-B C 111 III to see how they employed the Von Mises/ relev'e/ reflex-camber profile,simply bending the extra length up to clear the ground.
Final note: By taking the % Cd difference of where you are as opposed to where you want to go,half of that value will give your new HWY mpg multiplier @ 55 mph ( 88.6 Km/h ).
Happy modding!

Post-Script
 

I failed to mention,that for those,very daring and enterprising, who were to move their vehicles heat-exchangers to the rear of the vehicle,a la FORD Probe-IV,V,and configure your ducting likewise,you might drop another 0.011 from your Cd.
Truly scary efficiency!

Tail cutting and Cd variation
 

Basjoos was asked about mpg potentialities based on the length of his AEROCIVICs boat tail,so I thought I'd drag this thread back up as it is germane to the discussion.
Anyone familiar with the 'Aerodynamic Streamlining Template' can pencil in the values listed above in this thread onto their respective % position on the 'Template' for a quick reference as to drag potential vs boat tail length.

Thanks Phil.

A2 and ................
 

Thanks Vekke! For those of us without full-scale wind tunnels,these hard-science glimpses of real world testing really add to our portfolio and help substantiate or repute claims offered by others.
Really appreciate it.:D

Phil thank you for posting this (and Vekke for the PDF) but my left-brained art student logic is having trouble connecting the dots. Since the fronts of production cars are decidedly not template, and the boattails we build from them will not start at the 30% mark like the template does, how do we interpret the % points you posted? Also, is there a way to modify the baseline to fit our specific vehicles? I'd like to know how much I can get from 24" - 36" - 50" boat tails. (with 13* angles I can do a 100% tail and still lie within the length of a Ford F150 swb crew cab!) Thanks again for putting up with my relentless questions :)

for me, a real question is if I take an SUV, and add an 80 percent boattail to the end of it, what will my Cd be?

It's not that easy. If your SUV has huge side mirrors, roof rails/rack, mudflaps, wide tires, etc., then they will spoil the air going along the side/roof, so the Kamm extension will give you less gain. If the sides, roof and belly are smooth, then the same 80% Kamm can be used to its full potential and will give a larger Cd reduction.

it IS that easy.

the tail is by far the biggest effect. I wonder how much it is worth.