While Trebuchet03 shows us how to do things by the book with
CFD (computational fluid dynamics) to optimize a Kammback shape on his Jetta sedan, I employed the other kind of CFD on my brother's minivan:
Cardboard Fabrication with
Ducttape 
This is a variation of Craig Vetter's
CAD process: Cardboard Aided Design
This was an A-B-A on road test under perhaps the most ideal conditions I've ever encountered on my favourite test road. No wind, no traffic, perfectly functioning cruise control
, tight distribution in the results. I'll post the full details later, but here's the skinny:
Vehicle is a 2005 Pontiac Montana, long wheelbase version, 3.4L V6, automatic...
(More photos below in post #20)
This roughly slapped together Kammback, with fairly conservative angles (10 degree plan & roof taper), improved fuel economy by 3.7% in AAAA BB AA results.
If you add in the initial roof rack removal I did to get to the clean roof, the total fuel economy improvement was +6.6%. That's significant, and hopefully may motivate some of you van/trucklet drivers.
Absolutely nothing new here of course. Much attention has gone to studying and reducing the size of the wake behind trucks, since their cargo-carrying capability dictates a boxy profile with no taper at all. Researchers have known for decades that:
Aerodynamic drag can be significantly reduced with trailer add-ons that reduce the wake and increase the base pressure. - Source:
SAE 2000-01-2209
Edit: added the following, Nov 23...
Details of the test...
- Test speed: 88.5 km/h / 55 mph
- Cruise control set once, cancelled with the brake pedal between runs; "resumed" for subsequent runs
- Bidirectional averaged runs on a straight and nearly level test route; runs were abandoned & re-run where I was overtaken or caught up to another vehicle (aero interference)
- Wind conditions were perfect: none!
A runs, average of all: 33.72 mpg (US)
A runs, standard deviation (of average of bi-dir pairs): 0.35 mpg
B runs, average: 34.975 mpg (US)
B runs, standard deviation (of average of bi-dir pairs): 0.18 mpg
Improvement of B over A: 1.255 mpg / 3.7%
Roof racks on, Kammback off (one bi-dir run): 32.8 mpg (US)
Improvement of B over roof racks: 2.175 mpg (US) / 6.6%