[aerohead]

Conventional vans are for hauling,and are optimized for useful interior volume.So you get a lot of floor area you can stack stuff,right up to the ceiling.Its a cubist's dream come true! The down side,is that vans tend to be slab-sided,with very little plan- curvature ( curvature to the sides when viewed from above),and a relatively flat roofline.--------------------------------------------- This means that the van is basically a "full wake" vehicle.The cross-sectional area of the van,from the windshield back,is constant,at full projected frontal area,until the very back of the van.This means that the wake of the van is also the size of the full cross-sectional area of the van.Really big!!!!!!!!!!!!!!!!!!!!!!!!! ------------------------------ With the "bionic" car,you'll notice that the roofline curves gently down as it projects towards the rear of the car.Also,if you were above the car,looking down,you'd see also,that it was growing ever so slightly more narrow from front to back.This is where the Cd0.19 comes from.--------------------------------------- For your van,the air is disturbed from it's "rest" position,knocked out of place and accelerated to about 1.3-times the velocity of your speedometer,and since there is no narrowing of the body,the air stays completely out of equilibrium,until the van passes,where it tumbles into the void left behind the van,creating an enormous wake of turbulent air which cannot convert itself from its state of kinetic energy,to the calm,atmospheric pressure air it was before you came along.This in essense is your aerodynamic drag.--------------------------------------- And for your van there is no easy remedy,as the only way to get the air re-organized without turbulence,requires a lengthy tail.You could do all the mods you see for the Fed Ex concept truck.This would reduce the forebody drag,when combined with a full bellypan.You might see Cd 0.20.------------------------------------------- If your at say, Cd 0.38 now,and you could cut drag to 0.20,that's a 48% drag reduction,and would be good for about 24% better mpg at 55-mph,a little better at 70-mph,I don't think you could do better unless you cut into the roof and sides,and re- designed the rear 1/3rd of the vehicle.All your time trouble and material expense would have to be recouped in fuel savings.And you'd be losing interior space.--------------------------------------- A boat-tail will give you some savings,but the extra length can be a real issue when parking.-------------------------- you may want to do a ecomodder search for aero-mods # 11(aftbody streamlining/boat tails),there is some empirical data for you to peruse and may be of benefit to you.