Just to be clear, I don't think you can change the speed the cable rotates at. You can't put in a different cog to make it spin faster, assuming t's a standard series output: your transfer box looks odd, it's a Santana, isn't it?

You can change the clock in the instrument panel, but unless it was changed by the previous owner, you shouldn't have to.

There is a very small number on the speedo face - something around 1000 to 1250. This is the turns per mile and once you have this you can go on-line and check you have the right one. If you have larger tyres or 3.54 diffs expect the speedo to be about 20% low but not more. It can under read if it is dry but this also usually causes the needle to wave about madly as it sticks and unsticks. If you can get the number you can put an electic drill on (set to backwards) and check the speed. You have to know the drill speed and do some calcs but its a home job. The speed is set by the gears inside the speedo and the spring, do not be tempted to try to adjust the spring, its a job that never ends well. Its designed to look adjustble to ensure you break it and have to buy a whole new one, well that's what I think!

Thanks everybody for your help and advice the PDF file was very interesting and useful.
I had changed the speedometer unit 38000 km ago and it was factory original and new so I am sure it is not the unit. My problem right now is knowing what causes the problem, the gearbox or the drive?
The mechanics say that if it was the drive the needle would not move at all. Since it is moving they say gearbox is faulty. I don't trust the lazy bastards and it does not sound logical either.
The drive from the look of it, must transfer the rotation at a different rate to the cable. So, if it is defective it makes sense that the needle act irregularly. If it has gotten too big it might rotate the cable less (show slower speed which is the case). Of course the gear in the gearbox could be worn and rotate the drive less, but it does not take away from the earlier mentioned possibility. This is what my no-expert logic tells me and that's what I wanted to check with you.

Blackburn said:

Remove rear propshaft from handbrake drum and check the nut below it is torque up to 85ftlbs. This nut nips the speedo drive gear between bearing and output flange.
If that is tight you need to get speedometer checked the speed part is magnetically coupled and deflection set by a hair spring so not easy to fix.
The mistake some people make is to put to much oil / grease into cable this migrates into speedometer head and causes issues.
One last thing you can try take cable out of speedometer and put a tiny drop of very light oil around the section the cable goes into the bush there if dry will cause drag and affect speed reading. Have a look at the attached pdf.

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85Santana3 said:

I don't trust the lazy bastards and it does not sound logical either.

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Have you done the above as @Blackburn suggested. I had a similar issue and bought a new speedo drive pinion ready to fit to rectify the problem but never fitted it, I found the nut behind the propshaft to only be finger tight and once torqued up everything worked correctly.

Dag019 no I have not done that yet because it requires tools and lift which I do not have. I am planing to take it to a garage and do it there. Meanwhile I am searching online for a photo or diagram to see where is that nut is.

85Santana3 said:

Dag019 no I have not done that yet because it requires tools and lift which I do not have. I am planing to take it to a garage and do it there. Meanwhile I am searching online for a photo or diagram to see where is that nut is.

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It doesn't require a lift, there is plenty of space to lay underneath. The only tools you need are you need a pair of 9/16 spanners to remove the propshaft. I do not know what size the nut underneath but a large adjustable will tell you if it is tight or not even if this does not give you an accurate torque.

Couple of snapshots from manual and parts manual.
The item 5 on the manual is the nut and split pin that holds the gear 540004 nipped between the bearing 90217512 between part 266303. The inside of the speedo drive is not splined onto shaft and relies on friction grip between the two parts named. I have a speedo drive gear loose in my garage next time I am in will take pic of centre to show what I mean.

This has the makings of a solution, I'd forgotten the drive gear was only pinched. I'm fairly sure the nut on the flange is either 1" or 15/16 AF since I think that's the ring spanner I used. I had to change the seals and found that nut easy to undo (despite allegedly being at 85ft lbs) however the front one was more like 200ft lb so I suspect the drive torque loosens the rear and tightens the front.

Here is a link to speedometer worm gear showing internal of the gear.
https://rimmerbros.com/Item--i-5400...MI5ebyptHv5gIViLPtCh0Vdgp0EAQYBiABEgIlIvD_BwE