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Do I need a different speedo drive pinion?


RichardS
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I hope someone can tell me if I'm on the right track and whether there is a simple solution to this?

My 1973 GT6 Overdrive has a standard 3.89 differential but has, at some point, been fitted with a recondition gearbox and overdrive unit. It all works well but the problem is that the speedometer and odometer and trip all read around 20% too high.

The speedo head fitted is a Smiths 6211/00S 980. Is this the same speedo head fitted to all GT6's, overdrive and non-overdrive?

The actual vehicle speed at 2500 rpm is 44mph in top gear and and 54mph with overdrive engaged. I believe these figures confirm that the problem here is that my overdrive and speedo drive must have been specified to work with a differential which is around 20% higher geared than mine. As my diff is 3.89:1, I suspect that my overdrive/speedo drive is designed to work with a diff which is around 3.2:1

Is there a Triumph model which has a gearbox and overdrive unit which is interchangeable with the GT6 but uses a 3.2 diff rather than a 3.9 diff?

Looking at the Rimmers site, there is a speedo pinion part number 119100 for the non-overdrive car whereas the part number for the overdrive car is part number 506199. Does anyone know if I can simply buy the 506199 drive and swap out the drive pinion from my overdrive?

All advice appreciated.

Richard

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Hi,

 The speedo is matched to the diff. Overdrive or not will have no effect on the reading.
A different pinion on OD or non-OD? I guess its possible as factory install of OD meant a different Diff.
Mine reads 10% low. So following with interest.

980 TPM. Somebody will tell you what diff that is for.

Cheers,

Iain.
 

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1 hour ago, SpitFire6 said:

980 TPM. Somebody will tell you what diff that is for.

I'll tell you what it's NOT for - an overdrive GT6! I can't remember the correct TPM figure for the 3-rail with 3.89 diff but it's four digits. I think 11xx, and the 4.11 diff should have 1214 or something. 980 could be for a 3.27, non-OD spec.

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4 minutes ago, NonMember said:

I'll tell you what it's NOT for - an overdrive GT6! I can't remember the correct TPM figure for the 3-rail with 3.89 diff but it's four digits. I think 11xx, and the 4.11 diff should have 1214 or something. 980 could be for a 3.27, non-OD spec.

So it could be the speedo head that is wrong rather than the pinion?

I'm very unfamiliar with speedometer technicalities but if someone could tell me what the correct TPM is for a GT6 with overdrive and a 3.89 differential it would be very helpful.

Assuming that the error I have is directly related to the TPM then I would have thought that I needed a speedo head which needs about 1180 turns for every mile?

Although the rim sizes are now 14 inch, the tyre circumference is only 5 turns per mile larger than standard.

Many thanks for all the ideas so far.

Richard

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10 hours ago, RichardS said:

I hope someone can tell me if I'm on the right track and whether there is a simple solution to this?

My 1973 GT6 Overdrive has a standard 3.89 differential but has, at some point, been fitted with a recondition gearbox and overdrive unit. It all works well but the problem is that the speedometer and odometer and trip all read around 20% too high.

The speedo head fitted is a Smiths 6211/00S 980. Is this the same speedo head fitted to all GT6's, overdrive and non-overdrive?

The actual vehicle speed at 2500 rpm is 44mph in top gear and and 54mph with overdrive engaged. I believe these figures confirm that the problem here is that my overdrive and speedo drive must have been specified to work with a differential which is around 20% higher geared than mine. As my diff is 3.89:1, I suspect that my overdrive/speedo drive is designed to work with a diff which is around 3.2:1

Is there a Triumph model which has a gearbox and overdrive unit which is interchangeable with the GT6 but uses a 3.2 diff rather than a 3.9 diff?

Looking at the Rimmers site, there is a speedo pinion part number 119100 for the non-overdrive car whereas the part number for the overdrive car is part number 506199. Does anyone know if I can simply buy the 506199 drive and swap out the drive pinion from my overdrive?

All advice appreciated.

Richard

Was your caroriginally OD equipped? Simple way to tell is the commissin number has an O after the number.

You could simply get your speedo recalibrated, speedy cables and speedograph both offer such a service, and that would ensure accuracy. 

And I do feel your pain. I have a car with a std dolomite sprint axle, overdrive gearbox and the correct 1000tpm speedo. And that is 15-20% out.(low reading) I ought to try another speedo I think. 

 

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2 hours ago, clive said:

Was your caroriginally OD equipped? Simple way to tell is the commissin number has an O after the number.

You could simply get your speedo recalibrated, speedy cables and speedograph both offer such a service, and that would ensure accuracy. 

And I do feel your pain. I have a car with a std dolomite sprint axle, overdrive gearbox and the correct 1000tpm speedo. And that is 15-20% out.(low reading) I ought to try another speedo I think. 

 

Yes, my number is KE23600O although I didn't know whether that last digit was 0 or O but I learn something new every day on here. 🙂

From the paperwork I have going back to the early 1980's, the engine and body are still the original pairing so, yes, it probably came with an overdrive.

I did notice that a few years ago the mileage on the MOT records suddenly dropped from 90k to 30k but it seems highly unlikely that someone did 40k miles in a year so I'm guessing that a speedo was fitted from a non-overdrive car. There's probably a receipt for it somewhere in the records but there is so much paperwork that I haven't had time to look through it all.

Richard

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12 minutes ago, dave.vitesse said:

I can confirm the 980 turns/mile is for the 3.27 diff. The speedo for the 3.89 is 1152 turns/mile. In others words you have the wrong speedo. The ref number should have 1152 on it's end.

Dave

 

18 minutes ago, Nigel Clark said:

Here's a pic of the speedo in my 1972 overdrive GT6, with standard 3.89 diff. Sorry for rubbish phone photo with reflections, but you can see it's marked 1152 tpm. It's reasonably accurate, given that it's almost 50 years old!

Nigel

20200609_092805.jpg

 

2 hours ago, NonMember said:

Well, according to this US posting the correct TPM is 1184, which sounds familiar, and 1248 for a Herald. However, I've just checked my GT6 (Mk3 OD) and it's 1152. You may also find references to 1000 TPM speedos but I believe that's the late, single rail gearbox (Spitfire 1500).

Thank you all for your help. It's now clear that I need a 1152 TPM speedo.

There are a couple of classic Triumph specialists near me that I'll pay a visit to but any other suggestions would be most welcome.

I used to work in Carshalton with a Nigel Clark who drove a Spitfire in the mid 70's.

Richard

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24 minutes ago, Nigel Clark said:

 It's reasonably accurate, given that it's almost 50 years old!

Back in 2016 my RBRR co-driver brought a satnav along, partly because he didn't trust the 1972 vintage speedo. The GPS speed showed that the speedo was consistently over-reading by 5%, which didn't surprise me but got me wondering, as the trip meter seemed to be out by the same. A couple of months on, I had a poke around underneath and noticed that the diff had a GE prefix, which I knew to be a 13/60 Herald number. A check of the wheel-to-prop rotations showed that it was, indeed, a 4.11 ratio!

The car now has a 3.63 diff and, as you'd expect, the speedo is about 5% low according to GPS. So it would be surprisingly accurate if I had the "proper" diff!

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11 minutes ago, RichardS said:

 

 

Thank you all for your help. It's now clear that I need a 1152 TPM speedo.

There are a couple of classic Triumph specialists near me that I'll pay a visit to but any other suggestions would be most welcome.

I used to work in Carshalton with a Nigel Clark who drove a Spitfire in the mid 70's.

Richard

Hi Richard,

That was me, at Vinyl Products, Mill Lane, Carshalton in the mid-Seventies! I had a Royal Blue Spit Mk3 and have owned Triumphs almost ever since.

I will send a PM.

Nigel

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  • 2 weeks later...

late t,this,n, did,nt see it

 

Dont need a speedo  change really ,but maybe easiest option

seeing as ye gotta tek the  tunnel oot,which is a feat in its self.

 

but ye can alter the drive pinion int OD,or GB end

a 3.27 should have a 3-1 ratio. 

thats   3 turns of GB input shaft, t,1 turn  drive pinion shaft in 4th gear

thats  wid a 6 start  drive scroll and a 18 tooth pin

 

but they can be altered between 6-17   6-18  6-19  6-20

I altered mine this way, as speedo was  reading wrong  after use,n  diff tyres

 

Butt, it all depends wot drive scroll ye got in

could be a 5,6,7 one,  then go alter the pin  drive t,suit what yer speedo wants.

 

M

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  • 3 weeks later...

I'm even more confused by this speedo issue.

We removed the speedo drive unit from side of the overdrive because it was leaking gearbox oil. The oil seal on the pinion shaft was completely shot so we replaced the oil seal and, whilst we were at it, replaced the speedo drive cable as the old one had several fractures in the outer casing. We did not touch the speedo itself other than unscrewing the old cable and screwing in the new one, all done from behind the dashboard with the speedo still fixed in position.

When we took the car for a test run, rather than reading 20% fast, the speedo now reads only 10% fast, which is acceptable and can be lived with.

However, I do not understand how putting in a new seal and a new drive cable can reduce the speedo reading by 10%? I would have understood if it was now over-reading even more ..... but less?

Richard

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30 minutes ago, RichardS said:

However, I do not understand how putting in a new seal and a new drive cable can reduce the speedo reading by 10%? I would have understood if it was now over-reading even more ..... but less?

Richard

Unless the cable is slipping? It's just a thought, but you may find that the cable isn't properly seated so is producing less force at the speedo end than it should be. It's the only explanation I can think of, if nothing else has been touched. Possibly, if you drive it further, it may slip even more, or it may reseat itself in which case you're back to 20% over again.

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Hi,

 I would have thought lube or not problems would only show as an oscillation on the needle at speeds under 70?
Too much friction normally means not working or stripped gears?
Input speed of the cable is always the same on output or cable is broken?

The cable cannot cause an error in the display?

Cheers,

Iain.

 

 

 

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