Hi Duncan
I'm sorry I misinterpreted your reply. However I don't understand why, in the case of a Venturi , the pressure is the square of the velocity when the sensing port in the throat is at right angles to the flow so shouldn't be sensing any velocity component. My reasoning is that lift generated by an aircraft wings upper surface, which is effectively half a Venturi, is proportional to airspeed. I don't remember, a least in my aeroplane, ever experiencing a square law increase in lift with increase in airspeed although when fully loaded I wish I had.
I'm not trying to calibrate the ASI using a car but just to confirm the Venturi law and constant as I don't have a wind tunnel to hand. For checking the calibration of ASI's I use a published equation relating pitot pressure and airspeed together with a precision capacitance manometer and a certified ASI as a double check.
Clive