Speedo drive gears and ratio calcs for tyres

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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 20 May 2016, 13:58

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:
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Re: Speedo drive gears and ratio calcs for tyres

Postby muileach » 21 May 2016, 13:46

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.
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 21 May 2016, 14:08

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

Postby silverbullet » 26 Jun 2017, 14:06

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

Postby silverbullet » 28 Jun 2017, 08:52

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
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