Speedometer Strangeness!

I recently replaced the speedo cable on my Series 3, 109" Station Wagon, after the speedo started to behave erratically, waving around all over the place.
So - new cable I thought, easy.
Having replaced the cable, the reading is now good and steady. BUT - it now reads consistently about 10mph higher than I'm really driving! (Confirmed by someone in another car).

So - what can possibly be causing this? Replacing a cable that rotates roughly for one that rotates smoothly can't cause it to rotate faster - can it?

Any suggestions as to how to fix this?
Thanks
Chris
NOTE
- the speed reading was correct (I believe) before the new cable;
- no other changes were made to tyres, gearbox, diff ratios etc

Speedo needles are magnetically coupled with watch spring to return it.
Worn cables can cause uneveness in rotating primary part

Sounds like you have fitted a speedo from a SWB (88") landy.. the difference in wheel circumference between the standard tyres would account for this - I base this on the fact that I have LWB (larger tyres) fitted on my SWB landy and as a result she reads about the same under what I am actually travelling

If you look closely at the dial guage there are a series of four numbers which identify the ratio of turns between wheels and speedo - these should be the same on the speedo that you have replaced and the one you have taken out

Thanks for replies.
However, to be clear, I've not changed the speedo. It's the same one that's been there for the 27 years I've owned the Landy! And it used to show the correct speed - until, as noted, the old cable became worn and reading became erratic.
However, I have checked the numbers on the speedo to identify it - the four digits at the botton of the dial are 1344. Other numbers are - at bottom SNT-6209/08, in centre 579182.
Any futher suggestions welcomed.

Where did you get the new cable from? I think mine involved blue packaging and didn't work properly until I'd properly flattened the end that went into the speed housing.

Also, it might not have been your cable in the first place. The cable is driven by a worm drive that is nipped between the bearings on the output of the transfer box. If the nut on the end of this lot comes loose then it will slip. To check you need to take the prop off at the front and check the nut and split-pin is all nice and tight.

If it has been loose you'll probably find that you need to take your hand brake back plate apart, give it a good clean up and burn the oil off the shoes and replace the output oil-seal.

If you pre-order the oil seals, felt washer and split pin and bits off page 1H-08 of the parts catalogue it should be a fairly simple Sunday afternoon job.

Is possible that whilst it was vibrating needle has moved on spindle , what position does it sit in when stationary?
Also possible spring is weakened have a read of this might help.
View attachment speedo.pdf