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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 20 May 2016, 13:58
by silverbullet
I have done some digging in ETKA, the info was there all along (bar the actual colours used on the speedo pinions) I had to print out the text and will re-write it later on (it won't let you CTRL-V)
There are actually less variants than I realized, but by making "the ratios that never were" we could end up with more accurate speedo readings for oversize tyre users, may be of some interest to 16" TDI users... :shock:

Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 21 May 2016, 13:46
by muileach
Seems awfull complicated, I am running 215/75/15 tyres on 7in wide wheels, my spedo is in kilometers so gave up using the spedo and now use either the mph on the sat nav or use a spedo app on the phone.

Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 21 May 2016, 14:08
by silverbullet
This is really all being driven by increasing numbers of stripped speedo drive pinions and dwindling supply , along with a degree of ignorance as to what fits and why.
Resorting to mobiles and satnavs isnt an answer when you go for an MOT with a non-working speedo...
For the sake if working through the numbers, why not have a correctly calibrated odometer and (legally) accurate speedometer?
I now have a raft of data from ETKA and Aidan, to add to the original info compiled by Derek Drew on thesamba.com, just need to reformat it all in order to make some recommendations for aftermarket tyre sizes.

PS this is what results from 5 (relatively low geared) different final drive ratios and as many oem tyre fitments.

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 26 Jun 2017, 14:06
by silverbullet
This side project has got moving again. Made a mandrel to carry the worm wheels in a front diff housing, so the grearcutters can check the mesh and tooth form etc on the various ratios.
Concentrating on the 8:19/20/21 first as they are the most popular.Image

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 28 Jun 2017, 08:52
by silverbullet
I have edited page 1 to include the pinion colours that I do know are correct.

Speedo drive ratio = W

7/21 = 0.333 *
7/20 = 0.35 *
7/19 = 0.368 Brown
8/21 = 0.381 Red
7/18 = 0.389 *
8/20 = 0.4 Black
9/22 = 0.409 was also available
8/19 = 0.421 White
9/21 = 0.428
8/18 = 0.444 Olive/bronze
9/20 = 0.45
9/19 = 0.474 *
9/18 = 0.5 *

* these ratios were never made

Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 25 Sep 2017, 21:38
by silverbullet
Breif update: 8/19, 8/20 and 8/21 pinions should be in production soon.

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 16 Oct 2017, 13:00
by silverbullet
I have now been told that the pinions will be with me by the end of the month, so keep an eye on you favourite webshop ;)

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 03 Nov 2017, 12:02
by silverbullet
The first batch of 8/19, 8/20 and 8/21 bronze syncro speedo pinions have landed!
Keep an eye on your favourite Yorkshire T3 parts suppliers webshop for retail pricing ;)

They are a slip-on fit to your old shaft, advise fixing with hi-strength retaining grade Loctite or Bondloc product e.g. 620 or 638ImageImageImage

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 03 Nov 2017, 15:41
by silverbullet
Side note for fitting: the shaft should be completely free running in the alloy bearing sleeve when degreased, without any hint of tightness or being notchy or "digital"
If it is found to be tight, use of a sharp & full size 10.0mm reamer will be required to resize the sleeve bore.
Corrosion may be the culprit, I suspect theat the sleeves get squeezed down at the outer end and so lose the running clearance, which is only 0.05 on diameter (about half a humam hair)

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 07 Nov 2017, 19:10
by crazydiamond
I just cheat! I use a GPS speedo it runs either mph or kph and when I compare it with the satnav its spot on. When on the syncro with 205/80/16 Kumho AT tyres the vans speedo is fast over most of the range, so I just prefer the GPS, so when I run different tyres such as my BFG MT 215/75/15 it still is correct, without doing anything. Only problem is odometer but I live with that (lets face it a 2.3 5 pot running LPG is never going to be frugal)

Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 15 Nov 2017, 08:54
by Simon Baxter

Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 15 Nov 2017, 16:00
by silverbullet
I have had an enquiry for the 7/19 which is the sole 7 tooth worm pinion for AOM, AOL 37/6 (6.16) and also 14" ALM, ALN 35/6 (5.83)

So this could be the next ratio for the 16" posse, unless there are other priorities? Such as:

9/20 (7.00 R16)
9/21 (6.50, 205 & 215 R 16)
9/22 (195 R 16)
All for ADM AHU AHT 38/7 (5.43) diffs running the above tyre sizes.

Damn its confusing!

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 11 Dec 2017, 15:31
by silverbullet
A graphic demonstration of why you cant just use a pinion with the correct number of teeth, regardless of the driving wormwheel: because they are rather different diameters!
Plus some handy shots from ETKA listing the myriad diff, tyre and speedo drives...ImageImageImageImageImage

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 11 Dec 2017, 15:35
by silverbullet
...and the last few speedo pinions!Image

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Re: Speedo drive gears and ratio calcs for tyres

PostPosted: 11 Dec 2017, 23:16
by footstuck
I think I am possibly getting valve bounce at £87 :wink:
Just need to know the current Rotational value to calc how much less than £87 I need to prevent braking free of orbit. I also require a Temporary' safe value of maximum velocity, I can face the music with my chosen planetary expert when I have the funds available.
FD= 616 with .07 4th n 2058516's
Calcs to explain it, would be a good run through demo if any would be prepared please.
No naysayers at the result please :tsk
I intend to stay quiet close to the tyre size given


Electronic TDI
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