Hall effect   reliable but expensive
Magnet / coil    reliable and cheeep    but boring
Optics    they are going to get dirty
reed switch    probably will work   but is has been done ( yawn )


BETTER.....
Put mouse or squirrel or ferret on bungee cord below bike seat and  
force the creature to run on the back wheel.  This will help keep the  
back wheel clean.  Attach two metal pads to creature's chest and  
measure heart rate.  After calibration, this will yield a correct  
speed of plus/minus 10kph ( unless the creature is in a mood ).  This  
can be corrected for by having 3 creatures ( who are strangers to  
avoid excessive chittering ).  The average of the 3 creature heart  
rate will be accurate to plus/minus 5 kph I think and moodiness will  
be ruled out.

Tips :
Be sure to feed and water your speedometer.  Works best at room  
temperature.

Gus
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