Aerodynamics question. Airflow under car?

[BBBrad]

I had a search around and cant quite get a definitive answer. I want to know how much air flows under a car while traveling? I know that great effort is put into the front of racecars to lessen airflow under the car. What about your normal street car with 100mm of ground clearance? I know of stories of aw11's with the rear hatch unlocked, and the airflow pushing the lid up and up the faster you go.
So, lots of air? Or not much at all?

[iOnic]

http://www.autospeed.com/cms/article.html?&A=2455
http://www.autospeed.com/cms/article.html?&A=2456

[KinLoud]

Generally a road (factory spec) car will get some lift at the front of the car as speed increases.

A vertical air dam below the front of the car reduces airflow under car
- reducing drag (some exceptions)
- reducing lift as speed increases (perhaps creating some downforce)
- increases airflow through radiator.

Refer to Competition Car Aerodynamics by Simon McBeath
Has a good chapter about airdams, splitters and spoilers
It covers the effect on sedans (production type cars)

As for rear hatch lifting....
It will depend on pressure differential
Perhaps pressure below hatch is normal but pressure above is much lower due to airflow swirling around behind the MR2 rear window.