Originally Posted by
Ascend Charlie
No, they ain't uniformly spaced.
Blimey, you're right. Actually, there are all sorts of markings, uniformly and non-uniformly spaced ones.
Here's a good example from a Piper Warrior that shows that the distribution of marking spacing is neither constant, nor a linear function of KIAS.
Because it is a nonlinear function of airspeed, this confirms that there's a transformation of the displacement taking place a the mechanical level.
This could indeed be done through calibration of the membrane expansion, or using a lever that actuates a gearwheel through a guide.