'93 ZX twitchy speedo and wavy revs?

The speedo dial on my ZX has an annoying twitch, which makes telling the speed below about 50mph a very hit and miss affair. Upto 50mph the needle constantly wobbles buy around 10% of the current speed. So at
40mph it wavers between 40 and 44mph, and at around 50mph it finally stabilises and reads a constant speed (albeit about 10% faster than the speed reported by my GPS unit).
Also at higher speeds the rev counter periodically does a full wave from redline, to zero and back to the correct reading.
From memory it's done this for around 30,000 miles, and I'm just curious rather than concerned...
Regards, Jason.
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Jason Arthurs ( snipped-for-privacy@arthurs.org.uk) gurgled happily, sounding much like they were saying :

Speedo cable.

What engine? Petrol or diesel? Sounds like an iffy connection somewhere.
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My '93 ZX est. 1.9D did exactly the same thing. It turned out to be a faulty connection on the sender unit, which I believe is connected to the gearbox somewhere, IIRC....
Never did have mine fixed as I traded in for a bigger car and it never really bothered me tbh.

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specialist fitted a s/h sensor for about £2 and £10 labour or something equally miniscule and did it while my wife waited. Presumably the rev counter may be similar or does it get its info from somewhere else.
FWIW my speedo eventually failed and the original specialist I took it to diagnosed a duff speedo although the immobiliser had also failed. It turns out (as the specialist I now use told me) the fuse for the imobiliser also fed the speedo sensor. Anyway in between finding the second chap I drove the car for a year without a speedo because I had the foresight to remember the rev counter readings for various speeds.
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