We have only had SX bikes as we only do MX and they do not have a speedo. But I think there is a magnet on the front brake rotor that will make the speedo work. Checck that it is still there and fitted properly. @augiedoggie or @DW would know more about this.
Like @Axzon said, check the magnet.
Check your distance from the pick up on the end of the cable to your magnet. The symptom you're subscribing is usually caused by too big of air gap from the pick up to the magnet or the magnet is not passing through the centre of the pick up. (Unusual for that though unless you have fitted different wheels or discs).
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