The sensor is rather crude, being a magnetic reed switch, the final drive crown wheel carrying vanes which periodically shield the magnet and so make the switch drop out.

The result is a negative-going square wave, the frequency of which increases with road speed. The pulse width will of course reduce with speed, but I would not call it pulse width modulation.
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Empeg V2 60GB running v3 (alpha 11) and the autoupdated hijack from jemplode.