Speed Sensor Anyone???

[ardhuru]

While I do not have an answer as to why this is not used in aircraft (for example) it seems there must be a reason.

Wouldnt you get a wrong reading after factoring in the tail/head wind? The RPM measuring technique would work only in the absence of the above. For the same RPM, your actual speed would be different depending on whether the wind is aiding or resisting you.