Hi everyone, my first post here, I've read through this thread with great interest, some real cool experiments happening.
Thought I'd propose some non-mechanical ideas for measuring flow:
A venturi meter.
Along the lines of this:
http://hyperphysics.phy-astr.gsu.edu...s/venturi.html
Of course with something clever to observe the pressure differential as a signal.
Inertial meter, or coriolis, whatever you want to call it.
Best description I can find is good 'ol wikipedia:
http://en.wikipedia.org/wiki/Coriolis_flow_meter
Need some imagination to use it in a vehicle, especially a bike, but at least the engine could provide a good consistent vibration.