Speedo gear size

[Speedy Builder5Participant @speedybuilder5]

I have recently bought a Classic mini 1979 vintage. I should be asking on a mini forum, but getting technical – How do they do this ?

I am guessing that the car has been "Tweaked" and that the gearbox / final drive has been swapped out at some stage. Not a problem but the speedo reads !00Kph when the car has a GPS speed of 80 Kph.

Following he net, the speedo drive gear should have 16 teeth, but there are also 17 and 18 tooth varieties.

So it could be that I have the wrong gear or wrong speedo. However, moving on, How could you fit different sized gears without changing their centres.

this the typical sort of thing Speedo drive gear

Bob

[Speedy Builder5Participant @speedybuilder5]

[David SeniorParticipant @davidsenior29320]

The gear you show in the link has 15, 16, 17 and 18 tooth varieties. There are also 4 versions of the mating gear, 5, 6, and 7 teeth (not sure about the other). They give an angled drive.

In the early days it was to do with the final drive ratio. Then wheel sizes changed (not that the rolling radius changed much) and finally the gearing in the speedo head changed. A veritable minefield!

I sold mini parts for 25 years and I haven't a clue as to which combination you would need!

Now, if you want any lovely billet aluminium parts for the mini………..

Dave

[Grindstone CowboyParticipant @grindstonecowboy]

Probably your best bet is to get someone like Speedy Cables to re-calibrate the speedo head – I had this done for my Mini-based kit car, you just have to tell them how many turns of the cable there are for a certain number of turns of the wheel, then they sort it all out for you.

Rob

Edit – totally missed the coincidence of their company name and your username!

Edited By Grindstone Cowboy on 28/09/2023 18:12:50

[bernard towersParticipant @bernardtowers37738]

you quote kph not mph does that not increase the size of your minefield?. If your speedo is marked kph not mph do you not have to overlay that with mph markings?, if so the overlay could be graduated to suit.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Speedy Builder5 on 28/09/2023 16:52:19:

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[…] the speedo reads !00Kph when the car has a GPS speed of 80 Kph.

[…]

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Just a ‘calibration orientated’ thought …

Have you checked that it also reads 50kph at a GPS speed of 40kph ?

Not sure what species of Speedo the early Mini used

MichaelG.

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Edit: __ I strongly suspect that they used ‘Magnetic’ speedos [at least on all but the exotic models] … but I stand to be corrected

Edited By Michael Gilligan on 28/09/2023 18:54:51

[noel shelleyParticipant @noelshelley55608]

To accomadate different tooth counts the gear shaft is in an eccentric bush and is fitted to suit the gear wheel. Noel.

[Speedy Builder5Participant @speedybuilder5]

I think that is the next thing to do (check GPS at different speeds and graph it) – however I am not legal until I get a clean Controle Technique (MOT). Even that has its problems as strictly I can't even drive the car to the test centre.

Unfortunately its a radiator out to get to the speedo drive gear – just to check it out.

And back to part of my original question:-

How could you fit different sized gears without changing their centres.

Ps Yes, the speedo is KPH as we are living in Metric land.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Speedy Builder5 on 28/09/2023 21:14:31:

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[…]

And back to part of my original question:-

How could you fit different sized gears without changing their centres.

[…]

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I presume your post crossed with Noel’s

The pedantic answer being “you don’t … you do change their centres”

MichaelG.

[Nigel Graham 2Participant @nigelgraham2]

So either the different speedo drives had different cases, or it was done by tooth-sums. Different tooth-counts on each wheel but all adding to the same sum. Would that work?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by noel shelley on 28/09/2023 21:10:01:

To accomadate different tooth counts the gear shaft is in an eccentric bush and is fitted to suit the gear wheel.

__________________ ^^^ ______________

[Dave HalfordParticipant @davehalford22513]

Assuming 4th gear is 1:1 ratio stick it in gear, mark the tyre with chalk, take the plugs out and rotate the flywheel 1 rev and see what the wheel rotations are. That will give you the final drive ratio of your box.

That said all you really need to do is track down the correct gear set for your car and fit it.

[Pete RimmerParticipant @peterimmer30576]

Posted by Michael Gilligan on 28/09/2023 21:22:01:

Posted by Speedy Builder5 on 28/09/2023 21:14:31:

.

[…]

And back to part of my original question:-

How could you fit different sized gears without changing their centres.

[…]

.

I presume your post crossed with Noel’s

The pedantic answer being “you don’t … you do change their centres”

MichaelG.

The mating gear for the speedo is a worm so you could change the ratios whilst keeping the same centre distances simply by altering the worm pitch. A finer pitch on the worm would put more teeth on the gear and thus increase the ratio since one turn of the work advances the wormwheel one tooth.

As a matter of fact having a 5 tooth worm instead of a 6 tooth worm (the only two options from the vendor linked above) would make the speedo read 6/5 high giving a 96kph speedo reading at 80kph GPS reading, very close to what the OP reports as his error.

[Michael GilliganParticipant @michaelgilligan61133]

[Speedy] Bob

This looks very useful: **LINK**

https://www.classicminidiy.com/technical/gearing

MichaelG.

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Edit: __ See also the first two items on the side menu here:

http://www.guess-works.com/Tech/ratio.htm

Edited By Michael Gilligan on 29/09/2023 04:59:03

[Michael GilliganParticipant @michaelgilligan61133]

See here at 8:28

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Edited By Michael Gilligan on 29/09/2023 05:43:11

[John ATTLEEParticipant @johnattlee20632]

Bob,

A corrector gear box can be fitted between two speedo cables and Speedy cables or Thomas Richfield could advise or you could make one yourself. Suitable range of small gear available from RS. You could put a different scale on the speedo but be very careful not to interfere with the needle. The only problem is that the odometer will not read correctly and I always find that a pain. Don't worry about the MOT as there is no requirement for it to be calibrated. However, it does have to read correctly within certain percentages and it is a pain if it does not.

Speedy Cables can probably make the ododmeter read correctly. They can also zero the mileage if you have no confidence that anything like the correct mileage is being shown. It is quite fun to know how many miles YOU have driven it the vehicle in total, especially if you have overhauled it.

John

[not done it yetParticipant @notdoneityet]

There is a fellow on the morris minor forum that actually supplies bespoke speedo drive cable gears to change the turns fed into the speedo head.

In the Uk, the speedo error can be up to 10% optimistic but must not indicate a lower speed.

We are only talking about 200rpm at the speedo drive, so maybe some combinations are close enough to mesh adequately?

[Speedy Builder5Participant @speedybuilder5]

Thanks for all the comments and links. From Michael's link, I think I know what the engine is and correct output ratio of the gear box. Need to see if the TPM is on the speedo next.

Pete did you mean 15 / 16 tooth gear ?

And just for clarity, we are in France with a LHD French marketed mini and KPH. As an aside, during its life, the bolts are now a horrible mix of UNF/UNC and metric combinations – spanner night mare !

Bob

[Michael GilliganParticipant @michaelgilligan61133]

Regarding the worm/gear meshing of various combinations …

Until someone carefully measures that big bush, we will never be 100% sure … but meanwhile, I am happy to accept Noel’s statement at face value … it is eccentric, and therefore allows the centre distance to be variable.

MichaelG.

[Robert Atkinson 2Participant @robertatkinson2]

Not sure the Mini has an excentric bush. Due to the geometry of a worm and helix they can accomodate a tooth or two ratio range by adhusting the diameter and tooth shape. It changes the amount of engagement but as there is little power transmission in a spedo drive that is acceptable.

Don't know about the French test but in the UK the MOT only reqiures the speedometer to work. Assuming it is a regular annual test not a one of for approval after modification etc like the SVA in YK. There is no accuracy test. The construction rules say the speedometer must be accurate to -0 +10% when new. It also has to be "independant" so you can't use GPS.

IF accuracy is a requirement but it is over-reading can you just put a label on the speedos saying it over-reads by 20kph?

If you are of an electronics bent you could add a digital speedometer. There are plenty of RPM meter (same thing different calibration) designs and even modules out there. If the back of the speedo is plastic you can often pickup a signal from the rotating magnet inside using a coil attached to the outside.

Robert.

[noel shelleyParticipant @noelshelley55608]

My comment on accomadating various drive gear sizes with an eccentric bush was not in the context of a mini more Bedford 4 speeds of the 70s – the box that was always nicked ! Somuch so people would weld them in ! Noel.

[Michael GilliganParticipant @michaelgilligan61133]

On well …

MichaelG.

[Dave SParticipant @daves59043]

I seem to recall differing gear tooth counts can be accomodated by pitch shifting the gear.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 29/09/2023 10:54:06:

On well …

MichaelG.

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Sorry … that should have read Oh well …

[Pete RimmerParticipant @peterimmer30576]

Looking further into this is seems that the 6- and 7-tooth reference to the drive worm is actually 'starts', as in 6-start worm and 7-start worm etc. This makes any figuring-out far too complicated without parts to measure as the number of starts and tooth size affect the helix angle.

Given the above if I were faced with this issue would knock up a simple 5/4 reuduction gearbox for the cable. and use that to alter the speedo reading.

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