With the help of my lovely wife, I have made the air pressure measurements on my 914 at speed. The reference point was the ashtray in the dash. The ends of both hoses were buried in the center of 2" blocks of open cell foam to prevent air movement from affecting the pressure readings. I realize the reference point is not going to be at exactly zero, but as a reference it will show pressure diffrences between different parts of the car. The windows were rolled up, and the targa top was on. Here are the numbers, in kilopascals:

at 45 MPH

front center of front bumper: 0.21
middle top of front trunk lid: 0.03
inside front of front wheel well: 0.00
middle top of roll bar: -0.03
middle of rear window: -0.03
inside engine compartment (coasting at idle): -0.01
inside rear wheel well: 0.02

at 65 MPH

front center of front bumper: 0.44
middle top of front trunk lid: 0.02
inside front of front wheel well: 0.01
middle top of roll bar: -0.05
middle of rear window: -0.04
inside engine compartment (coasting at idle): -0.03
inside rear wheel well: 0.03

After our last measurement, we rolled down the driver's side window and drove at 45 MPH to see what difference that made on the reference point. The rear wheel well measurement went up from 0.02 to 0.07, which means that at 45 MPH the pressure at the ashtray is 0.05 kPa lower with the driver's side window down.

Whelp, there's my little numerical contribution to the 914 community.

--Royce