Speedo reads high

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johnhannam
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Speedo reads high

Post by johnhannam »

I have a 948cc Minor convertible ( smooth gearbox case) and the lay shaft wore out one day.. I picked up a nice secondhand (smooth gearbox case) gear box and found a local garage to fit it. I was happy with the result. The replacement box changed smoothly and did not leak.
BUT
The speedo reads about 20 mph low all the time as checked with my sat nav. What can I do? I have contacted various Mm experts who suggest,
1) getting the gearbox recalibrated by Speedy Cables?
2) moving the speedo needle physically 20mph forward myself?
3) fitting a replacement 948 speedo drive from my box?
4) cleaning and lubricating the gearbox cable thoroughly?
5) Replacing the speedo with a similar one with the same number?

I would welcome any suggestions before I get done for speeding!
Regards,
John Hannam
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Re: Speedo reads high

Post by Sleeper »

No 2.

John ;-)
johnhannam
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Re: Speedo reads high

Post by johnhannam »

Another error. For get gearbox recalibrated read get SPEEDO recalibrated by Speedy Cables
simmitc
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Re: Speedo reads high

Post by simmitc »

Did you check the original box against the satnav, and was it correct?

I would start by swapping the drive from the old box into the new box and re-check.

Logically, nothing else has changed, so the cable should not need attention, getting another speedo with the same numbers should give the same reading. You could get a specialist company recalibrate the speedo to match your current drive. Speedos are delicate instruments and whilst you can service them yourself, there is plenty of scope for getting it wrong.

Another option: Stick some labels on the speedo to show the currently correct places for the needle.

The topic title says "reads high", he post says "reads low". Whichever, it's still wrong, but I am curious to know which it is?
oliver90owner
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Re: Speedo reads high

Post by oliver90owner »

The topic title says "reads high", he post says "reads low". Whichever, it's still wrong, but I am curious to know which it is?

Me, too. Does this mean he is travelling at an indicated 20mph while stationary? Or is he still stationary, according to the speedo, when actually travelling at 20mph?

One simple check, if things are that far out, would be to check the odometer. I would be doing that as a first check to see exactly what the problem is - without guessing which of any one of the 5 alternatives might solve the issue.
James k
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Re: Speedo reads high

Post by James k »

Judging by the talk of moving the needle forwards to counteract the problem, I'm guessing it's the title that's wrong and the OP means it's reading low.

I'm also assuming that the needle sits at zero when stationary but moves up the scale too slowly. Is that right?

Assuming this is the case, as Simmitc has said, the only thing that's changed is the gearbox. If it has a speedo drive for a lower diff fitted then it will read low. A lower final drive means the wheels will be spinning faster compared to the gearbox. With the corresponding speedo drive fitted, the speedo will "think" the wheels are turning faster than they are.

Again, as Simmitc suggested, swap the drive over from your old gearbox, assuming you still have it.

Let us know how it goes.
johnhannam
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Re: Speedo reads high

Post by johnhannam »

Hello All,
Thanks for the helpful replies.
Apologies for my mistakes in the original text. As you guessed the speedo reads low. That is the speedo reads 30 when the car is doing 50. Also the speedo reads 0 when the car is standing still but as the car speeds up the indicated speed is consistently low.
It appears your vote is that I should look for another drive from the gearbox to the speedo cable. The old drive was discarded by the garage that fitted the replacement secondhand box.
My question is now does anyone have a drive for a 948 box to the speedo cable.
Thank-you for your patience and understanding.
Regards,
John Hannam
simmitc
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Re: Speedo reads high

Post by simmitc »

Club spares?
liammonty
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Re: Speedo reads high

Post by liammonty »

I'm sorry to spoil the fun, but I don't think you'll be able to replace the drive for another one and see any difference. The reason is that it meshes with a drive gear on the 3rd motion shaft in the 'box, so short of stripping the 'box and replacing that too, you won't be able to alter the output of the drive. You will only be able to replace like with like in terms of the drive that screws in to the outside of the gearbox.

I wonder if the fault might actually be with the speedo itself, and the change of 'box is a red herring? It would be good to know if the speedo fitted is correct for the car, and what the diff ratio is.

Do you know what the gearbox came out of? Could it have been a Midget, for example? I'm not sure, but I wonder if they had different speedo drive ratios to the Minor boxes. If that is the case, then the options are either replace the gear on the 3rd motion shaft as well as the speedo drive on the outside that meshes with it, or, get the speedo recalibrated to match what you've got.

Finally, does the speedo read 20 mph under from the outset and at all speeds, i.e. not registering until over 20 mph? Seems it's more likely to read a percentage under, which will give more of a clue as to what the 'fault' is :-).
oliver90owner
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Re: Speedo reads high

Post by oliver90owner »

I wonder if the fault might actually be with the speedo itself, and the change of 'box is a red herring? It would be good to know if the speedo fitted is correct for the car, and what the diff ratio is.

Back to my post earlier, then? 🙂🙂 A relatively simple check to determine the difference. The odometer is the only direct mechanical reading/indication within the speedo head. I suspect the difference is actually about 23% - a linear error - not a fixed error of indicated speed.
johnhannam
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Re: Speedo reads high

Post by johnhannam »

I will report back on the result of this conundrum
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