My speedo needle waves around, so I have no idea of my speed.

Is there an easy fix?

How much does it wave around?

Make sure both ends of the speedo cable are tight.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

I'd say 10 to 20 MPH.

I have checked cable tightness.

Mine does this too, on mine it's the friction to turn the odometer wheels of my old Moke 140kmh speedo.
When the ratchet pawl engages the wheel, the needle drops, as it retracts the needle rises.
I need to pull the speedo out and lube the bits but haven't got a round tuit.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Can also be a sign the cable is sticking in it's sheath, had two fail a few months after showing this symptom.
Both had burns from getting too close to the headers.

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Too many cars, and too little time.

another cause is that the magnet plate that's driven by the speedo cable lightly touches/drags on the cup that the needle spring is attached to - there's a little bush under it that wears, when it does that the clearance is reduced and the needle jumps around....

new cable first!

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did I tell you that I won a trophy?

New cable on the way. Will advise results.