Why might my speedo be failing?

[TrickyMicky]

Maybe he has decided to stop pulling long 'wheelies' now that he's realised the speedo only works when the front wheel is turning???

[K2-K6]

If one breaks down near the wheel end, then the "floating" upper part can "snaggle" the still fixed end to give intermittent drive sometimes.

Had a once in a lifetime (well I expect it to be) headlight builb low beam go out at over 5000rpm on a Kawasaki triple, but on again below that, which turned out to be the bulb's filament detached from one of the main posts, to "wiggle" up out of contact as vibration increased.

Less vibration or static, and it remained on 

[Athame57]

If one breaks down near the wheel end, then the "floating" upper part can "snaggle" the still fixed end to give intermittent drive sometimes.

Well actually that's what it has been doing today, working when it feels like it. Some meddling with the cable on the move ( bad idea!  ) can get it going again. I haven't pulled the cable out for inspection yet, I'm getting a bit cheeses off with it.

[florence]

I've been through so many speedometers it is beyond belief.  One wouldn't go above 50MPH, another developed an alarming squeal above 30MPH, another, the odometer stopped working and another was so vague, it waved + or - 20MPH whenever it felt like it.  I have consoled myself by coming to terms with the fact that speedometers are largely a cosmetic addition to the area above the headlight and now rarely look at it 

[Athame57]

I have consoled myself by coming to terms with the fact that speedometers are largely a cosmetic addition to the area above the headlight and now rarely look at it 

Well, actually I've thought of giving up with it for a while too! 

[Rayzerman]

You may want to dive in deeper and do a full refurb........
https://www.youtube.com/watch?v=PLU2sCFx6qU&list=PLSL5PUw3f6df2_9SmyiV7tF9XH_jnxlhj&index=14

[K2-K6]

Undo the cable from speedo end, zip tie it so you can see it when riding, stick a tape "flag" to the cable end and then see if it turns all the time.

Separate the system into two parts, then look at each section on that analysis.