getting out of ground effect...
 

Reading various articles on how streamliners, when touching the ground, have worse fuel efficiency than they do in free air - ie http://ecomodder.com/forum/attachmen...1&d=1247946422 (ie the half body at ground plane is over 2x as bad as the streamliner in free air) and having seen a solar streamliner body design that I can't find a link to right now, but which had like a 0.09 coefficient of drag not by hugging the ground but by letting the air flow under it better (it may have been 12-15 inches up but it was also going very very slow like 20mph, i'd assume as speed goes up you'd need more clearance) i'm wondering at what point of height/speed/other factors it makes sense to start letting air flow under the vehicle again?

Low ground clearance is hard to drive with in a world of speed bumps and potholes. And some of my projects wanting to be based around pickups tend to sit up higher already. When is the belly pan and a lift better aero than a ground effects kit and lowering?

My weekly driver had to be raised from where I wanted it, to where it would clear *most* speed bumps. The panel van I am going to put back on the road will be stock height with wheel spats like your solar streamliner surely had. Differing purpose.

Have you researched Luigi Colani? He has a handle on aerodynamics like Nikola Telsa had on electricity, they can make them do tricks. Here is a car based on the Water Strider than is an inverted airfoil:
http://ecomodder.com/forum/member-fr...wu5wo1-500.jpghttp://ecomodder.com/forum/member-fr...-c-form-02.jpg

I think Colani is more about whacked-out appearances than real aero.

Here's a link that might give you some info on optimum height. It's the section on ground effect in the Solar Car Primer and gives the formula for calculating optimum ground clearance for lowest drag. For a 6.5 foot wide teardrop shaped vehicle the optimum ground clearance is about 20 inches.

A solar car primer - Eric F. Thacher - Google Books

I'd say both, as they are both important. :)
http://ecomodder.com/forum/member-fr...629-images.jpg
I especially like the 3-bladed spinning windshield wiper on his M-B trucks—now that's branding. And why shouldn't locomotive wheels run negative camber like wheelchairs?

Been waiting for this question sort of. Its all about air pressure and aero form.
The mass still pushes through air at varying rate as determined by air density, speed, and resistance. Turbulence is caused by and also creates air pockets that become part of the total mass or follow behind your mass.
Damming frontal air instead of spoiling it effectivly causes vaccume under front of car.
The better approach is to slice through the air if possible distributing compressive force equally about the mass creating as little disturbance as possible.
disturbance and pressure pockets are sorta like parachutes to the aero effect.
I am sure some more educated mathematician here has a formula to determine how much air must be able to clear the bottom of vehicle but im thinking 2:1 would be good since most vehicles are designed for air over effectivly raising ground pressures at tires and reducing true aero.
Ok hows that clear as mud?

ground
 

*All bodies essentially 'double' in drag when in ground proximity.
*The half-body,without wheels, would be as low as Cd 0.08 vs 0.04 in free-flight.
*The GM Sunraycer was measured at Cd 0.089 in ground effect with wheels and wheel fairings.NUNA solar racers are in the Cd 0.07 range (You must consider how a motorist would get in and out of such a 'car.'
*If you look at Hucho's drag tables you find that a conventional motor vehicle would require an 11-foot ground clearance before significant drag reduction could be affected.
*From research published on the lowest drag,highest fuel economy concept cars constructed,the car is lowered with active suspension and raked for lowest drag/frontal area/low C.G./favorable C.P.(this concerns a real-world 'door-slammer' which a motorist could walk up to,open a door,get in or out unassisted).
*Aside from Bochum University's current solar racer,most other 'cars' require a crew to assist the driver's ingress/egress.