Will an almost ground-scrappin air dam and splitter reduce drag?
 

It is clear that a low ground clearance airdam and splitter would create downforce (or at least reduce high speed lift).

But would it also reduce reduce drag?
The dam would increase front area (thus potentially increasing drag) but would the reduced underbody flow compensate for this area increase to reduce overall drag?

A: Depends on the vehicle.

The good news is it is relatively easy to mock something up on your vehicle (CAD - cardboard aided design) and test it. (Stopwatch results of multiple coastdown runs with the dam on / off / on immediately following one another in identical conditions will reveal your answer.)

For example, see: Airdam - suprise result! - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

No splitter, and not quite a ground-scraper, but you get the idea. (You also see that it apparently did not help his second vehicle, thus the "it depends" statement.)

Will an almost ground-scrappin air dam and splitter reduce drag?

I actually tried this on my rabbit and it literally increased drag. The car had side skirts at the time and the vacuum under the car compressed the suspension a couple inches and drug the splitter on the road.
So resting height and driving height need to be considered.

bondo's beauty
 

bondo created an exquisite composite airdam for his F-150 and after tests realized that its extra depth was cutting into mpg.After raising,mpg came back.
Hucho's got some research covering this topic in his books.

A couple inches? Wow that's downforce!
At what speed was that? 55mph?

So it seems as though the air dam can be bad for smaller cars, but looks like it can help trucks that sit up higher, correct?

Trucks usually have suspension components that stick down lower. The rear axle, for instance. It becomes advantageous to push more air to the sides than you'd do with a typical McPherson setup which tucks up farther in the chassis.

My tiny little car certainly seems to benefit from a 5 inch airdam. (Still 3 or 4 inches between it and the road. I think, like the Mighty Mira, it's reducing the drag/visible frontal area of the front wheels, which I assume may be greater in proportion to the size of a small car. But in the case of my VW Station wagon which sits lower to the ground, I could see no difference.

http://i278.photobucket.com/albums/k...dua-airdam.jpg