[stcyrwm]

*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.

Thanks that is really simple and helpful! So going from the .218 Cd bus to the .131 Cd bus gives 60% drag reduction and 30% increase in mpg at 55 mph. Makes it seem worth it to do the reduction and have a liftable roof to get the space back, hmmm....

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*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.

Wow, okay so 80mm/3.15" is a much smaller radii than I was thinking was needed, which makes it seem much easier than what they do on an airstream for example.

Is that the same for the radii on the top edges of the sides before the taper???

And yes, I understand re closing TV to TT gap as NASA did.

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*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.

The compound build seems doable but the issue of liveable space usage seems harder to overcome. 5 degrees though seems fairly easy.


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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.

Okay I think I got this too.

Thank you, thank you, thank you!!!

I appreciate you and everyone who has contributed here so much,
Bill