[aerohead]

*A rule of thumb is,for each 10% drag reduction,you'll see a 5% increase in mpg; compared to the original Cd.
Example:lower your Cd from 0.30,to 0.15 = a 50% drag reduction,yields a 25% increase in mpg at 55-mph.
At higher velocity you'll see above 5% per 10% drag reduction.
--------------------------------------------------------------------------
*As to leading edge radii,the 'experts' say 'There's no magic radius.'
An 80mm radius is a safe bet,but there is an optimum,that's in the seminars,based upon a percentage of the square-root of the frontal area.
The ideal is,to provide articulated gap-closures,which seal the gap between the tow vehicle and trailer,as NASA did with their 18-wheeler in 1980.
--------------------------------------------------------------------------
*As to taper,that will depend on the level of difficulty on your build.Organic,compound curves are the 'best',but most difficult to fabricate.
For simple angled surfaces,the optimum angle will be a function of the length of the panel surface,compared to host structure.
For the bus diagram,all those angles are 5-degrees.
For the Ahmed body,you can see a sweet-spot angle,and then the impact of plan-taper,plus adding a diffuser.
Hucho/Buchheim and Mair say don't ever go steeper than 22-23 degrees.
I've been promoting the aerodynamic streamlining template,as it's a proven form,for 97-years now.But again,it's the most difficult to build.
--------------------------------------------------------------------------
As to boat-tail truncation,Kamm recommended chopping the tail off at 50% of frontal area,for urban driving,then Fachsenfeld advocated adding back some of the tail (or all of it) with an inflated portion,only for rural highway driving.
At the wind tunnel,the full-tail template body registered Cd 0.1209.When the stinger was removed at the 50% Af location,the drag rose to Cd 0.148.