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3.63 diff in Vitesse, speedo correction?


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Sorry well covered topic but I cant find a definitive answer on what my options are to correct the speedo reading when putting a 3.63 diff in my non-OD 2L Vitesse. Is there a chance of changing the gearbox speedo drive and if so how do I identify the gears I need? Thanks

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triumph did not do many alternative speedo pinions they just specified different TPM speedos  big saloon /TR boxes have some alternatives but not much for the small chassis gearbox

if the speedo is fitted  mark 52.8 ft on the road  accurately push car this distance , fit a card needle to the speedo cable inner and count the turns it makes 

multiply this count by 100 and you have the TPM you need to get another speedo or get it recalibrated 

 

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When we put a 3.63 in a Herald, my brother got a circle of card just larger than the speedo and marked the correct speed points on it.

The error on a Vitesse will only be about 5% which is way less than the misreading I've seen on most of my old Triumphs (although, bizarrely, the GT6 appears spot on once corrected for the diff change, which is what led me to "have a look" and discover that it had a 4.11 diff fitted for many years).

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If it was just the switch to a 3.63 diff, I wouldn't bother changing the speedo. As has been said, the effect of this one change is well within the (in)accuracy of the speedo anyway.

However, with larger tyres giving a greater rolling radius and higher gearing, added to the 3.63 diff, turns per mile may have changed by more than 10%, so recalibration is the sensible option in my opinion.

Nigel

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  • 3 weeks later...
  • 4 months later...

I found a battery powered GPS speedometer that you can set on the dash. Supposed to be very accurate.

It is "technically" a radar trap indicator devices that you download all known traps into it, supposedly legal and all but the upshot is it tracks your speed...

We have a lot of 30 kmh zones here and my speedo bobs up and down so I am not sure how accurate it is...

I have not bought the device yet but my plan is to use it instead of getting the speed recalibrated...

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if the error is reasonably linear  a small move of the needle may give you all you need to stay within the tolerances of speedo's

they must not read slow but read fast used to be 10% + 4mph when i ran a tacho centre in the factory ( the only truck factory to get UK approval ) 

all long gone so that spec may be a bit historic and havnt looked at Construction and use Rules for some years 

Pete

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The nearest I can find online appears to be in the region of -0 to +10%+2.4mph current, before prosecution for exeeding any precribed limit?. Which would suggest that Pete was near enough correct. 30mph would equate to 35.4 under those limitations

 

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A Vitesse came with the 4.11 diff, which is 16mph/1000rpm, while a 3.63 diff is 18mph/1000 (Both in top gear - direct drive - and assuming no great change in the rolling wheel circumference).

That is a 12.5% difference, so you might not be within the allowance for speedo error, that is usually 10% on speed cameras.

Pete is correct (of course!) about how Triumph and all UK manufacturers matched their transmisison and speedos.     Somewhere like Speedy Cables can rebulid a speedo to suit a different gearing,   

But I believe that US makers fitted diferent cogged speedo drive pinions to the gear box, so that a modifier could choose a different cog if the gearing was changed.

John

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1 hour ago, Pete Lewis said:

doesnt matter when were all electric we wont get that far anyway 

There is currently a thread on a facebook site generating a Huge amount of "opinion" on that subject. "Range" being the "BIG" objection to moving to E-V`s followed by Charging times and Grid capacity. I suspect that eventually all MPH will become GPS generated. Not that it will effect "our" cars unless or until some genius come up with a "conversion"?. Current issue of P-C is focused on "electric conversions" to Classics.

Pete

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on commer gearboxes the speedo drive was an eccentric housing so with a vast aray of transmisions and tyres you had a whole range of pinions and a fixed 5 start worm  so bigger or smaller pinions you just rotated the eccentric to give the right mesh and kept a fits all speedo , but a dozen pinions 

not rocket science 

then they invented electronic units you just flip the switches to set /match the gearbox output (W) 

Pete

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On 19/02/2021 at 13:16, JohnD said:

 

But I believe that US makers fitted diferent cogged speedo drive pinions to the gear box, so that a modifier could choose a different cog if the gearing was changed.

John

The later boxes and J type OD used different speedo drives in the gearbox. The speedos are all the same TPM (1000?)

Same system used in other makers gearboxes. Match the speedo drive to the diff.

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just playing with a speedo where the oddometer rolls are not indexing    and quite surprised to find the drive worm is loose on the main shaft 

no wonder its failed    but a simple solution will be spa glue  , it would need a drag to pull it off and being plastic thats going to wreck it 

if it wont stick its off to speedy or JDO as another repairer  www.jdmo1.co   worth pencilling in your little black book

Pete

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1 hour ago, Pete Lewis said:

if it wont stick its off to speedy or JDO as another repairer  www.jdmo1.co   worth pencilling in your little black book

Pete

I'd just fit a different speedo, probably much cheaper... or cannibalise another one. Herald ones are two a penny online.

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