how accurate do you want it to be?
(smart ass crack...cant you tell when youre going fast or not?
top speed of 90, if that, seems a bit pointless)
pushie speedo will work...if you can find one that will let you calibrate to ANY wheel diameter... (and a 12 inch pushie rim is not the same diametre as a moto 12inch rim
...therefore, wont be very accurate...)
what diameter is your front rim? and got any mates that are into electronics? the calculations are fairly easy... 2 pi R... then every revolution covers that distance. then a hall effect sensor mounted somewhere to count the revolutions(just like a trundle wheel
) and then some fancy circuits to measure number of revs per second, and display said measurement as an actual speed... there will even be circuit diagrams on the net somewhere, im sure
now does it seem worth while? no, i didnt think so... stupid bling, my man...
now if you were riding on the roads... you wouldnt even look at the speedo, cus soon you get used to what the engine sounds like in certain gears at certain speeds.... my oldbike was good that way... 4000 in 2nd was 40, in 3rd was 60, in 4th, 80, and in fifth, 100... speedo cable had broken... and once you know what 4000 rpm sounds like... tis easy
another thing is...you should be watching where youre riding, not at a speedo....