In the 4th link BigBlock posted it refers to a air pressure differential gauge magnehelic, that seems useful for us backyard aero wannabes. They seem readily available and in the under $125 range.

Anybody have any real world experience with these gauges?

Like, wouldn't a -2.5 to +2.5" of water be a better choice?

Is That enough range?
How sensitive are readings to how pressure tube opening is oriented?
Where on a car is the reference pressure obtained? Inside the car would seem rather variable, by speed, windows up or down, defroster on off, etc
Is an analog better then the slightly more costly digital gauge?

This is the page that caught me eye.
http://www.dwyer-inst.com/Product/Pressure/DifferentialPressure/Gages/SeriesDM-1000/Ordering

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.