[instarx]

You have both answers here: pressure is best measured with a manometer or magnahelic gauge inside the car in an easily observable location, with one piece of rubber tubing going to the location you want to measure the air pressure. Don't point the end of the tube directly into the air stream. Manometer is cheaper, magnahelic is easier.

Pressure alone is not a good indicator of airflow. In fact, high pressure often indicates low flow because the path is restricted (pressure is higher in front of a filter because the filter restricts the flow of air). Airflow is better measured with a pitot tube or a hot-wire anemometer. A pitot (pronounced "pee-toe") connects to a manometer or magnahelic inside the car with two rubber tubes. The hot-wire anemometer would need a sensor cable long enough to travel to the location you want to measure. Pitot tubes and anemometers are not cheap, but not terribly expensive either. The anemometer is direct reading for velocity, you have to do some simple math with the pitot tube.