Speedo drive gears and ratio calcs for tyres

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Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 27 Jan 2016, 22:07

Part 1

This subject was tackled on the samba a while ago, but wasnt really written in English or using the correct terms, so I thought that I would have another go at it!

I am doing this in several parts on my phone so give me a chance to get it all posted before commenting!

The problems: non-standard oversize tyres make the speedo read well out, sometimes making it read illegally low; ageing plastic speedo drive gears failing due to poor lubrication of the speedo pinion shaft (it siezes and gets shredded); a range of different speedo gears fitted dependng on original (M-code) tyre size option and diff ratios.
So its complicated but not impossible to sort out correctly, especially if the near-unobtanium speedo drives are remanufactured in more durable materials (more on that later) for when front diffs need refreshing.

In brief: there is a hardened steel worm wheel with 7,8 or 9 teeth that runs at propshaft speed and drives a plastic-headed pinion gear with anything from 18 to 21 teeth. The latter drives the speedo cable.

The magic number on your speedo dial face states the number of turns of the speedo cable needed to record a distance of 1 mile exactly.
(Rather usefully for an mph speedo this is 1600 and with one mile being ~1609 metres it's a convenient ratio)

So... the speedo cable turns at anything from 7/21 (0.333) down to 9/18 (0.5) times propshaft speed.

The wheels turn at 7/32nd (4.57 diff) down to 6/37th (6.16 diff) of propshaft speed.

If you know the rolling circumference of your tyres (3.142 x rolling diameter) and your diff ratios, then you can work out precisely which speedo drive gear set you need for a truly accurate reading on your chosen primary* tyre set.

*only really makes sense for tarmac tyres but more than a few people have a set of play tyres

Part 2 follows with lots of numbers!
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Re: Speedo drive gears and ratio calcs for tyres

Postby syncroandy » 27 Jan 2016, 22:18

Cable-driven speedo's are so 1980's.
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 27 Jan 2016, 22:32

Part 2. This is the complicated bit made easy.

1600 is the magic number for miles.

Work out how many times your tyres have to turn to cover one mile
E.g for 185/82R14 this will be 1609m/2.01m or near enough 800 wheel revolutions.
http://www.club80-90syncro.co.uk/Syncro ... ulator.htm

These tyres and 4.86 diffs throw up the following calculation:

1600 / (4.86x800) = 0.411

This falls midway between an 8/20 and a 8/19 speedo drive (0.4 and 0.421 respectively), so VW and SDP must have optimized the speedo drives based on real-world testing with various tyre sizes.

I will take this opportunity to apologize for not referencing the OEM speedo pinion colours, any remanufactured gears will probably be from hard brass and I will also be recycling the original (hardened steel) shafts because they are very good quality!

Side note: the pinions have very different tooth profiles. The 18-tooth pinion for a 7-tooth wheel is not the same as that for an 8 or 9-tooth wheel (and may go a way to explaining some of the stripped speedo drives out there)

This means that all replacements will be marked with their ratio amd should only be run with their correct worm wheel.

Its a work in progress.
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 27 Jan 2016, 22:37

Andy, really! Couldn't even wait :roll:
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Re: Speedo drive gears and ratio calcs for tyres

Postby max and caddy » 27 Jan 2016, 22:38

syncroandy wrote:Cable-driven speedo's are so 1980's.

So are T3s...that's the whole idea..
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 27 Jan 2016, 22:38

...and you still need functioning and correctly meshing gears to drive a modern speedo pulse generator!
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 27 Jan 2016, 22:43

Here is a boring list of the theoretical drive combinations:

7/18 = 0.389
8/18 = 0.444
9/18 = 0.5

7/19 = 0.368
8/19 = 0.421
9/19 = 0.474

7/20 = 0.35
8/20 = 0.4
9/20 = 0.45

7/21 = 0.333
8/21 = 0.381
9/21 = 0.428
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 27 Jan 2016, 22:56

While looking at the final drive ratios, there is a strange omission:

7/32 (4.57)
7/34 (4.86)
7/38 (5.43)

6/35 (5.83) the tall 16" ratio
6/37 (6.16)

Why is there no 7/36 (5.14) or is it a rare one like the 6/35 ?
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 27 Jan 2016, 23:17

PPS a note on gear meshing:
Part of the shredded speedo pinion phenomena may be due to them being swapped for the perceived "correct" ratio without the knowledge that the driving worm wheels have different diameters.
Get it wrong and that plastic pinion could be hanging on by the tips of its teeth, or forced hard into mesh causing it to overheat, seize or melt...
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 18 May 2016, 15:05

Just to bump this one up the page following a recent pm. I will be revisiting this matter fairly soon, once I have time to talk to the gear-cutters.

Can someone give me an opinion as to which ratios we need most urgently i.e. the widest application?

I don't want to lay down say 20 of each, only to have a load of "dead" gears on the shelf forever.

Maybe some further calculations to optimize for the most popular "alternative" tyre sizes would be a good idea?
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Re: Speedo drive gears and ratio calcs for tyres

Postby Aidan » 18 May 2016, 17:59

there are still three available from classic, on brickies website, which is one each of the z=7, z=8 and z=9 variants - you'd have to look which ones they are, but it includes the rare 14" 5.83 final drive with iirc 185s
for stock UK vehicles and most 14" european vans the z=8 is by far the most most common and there were 4 variants, 18, 19, 20 and 21 - 19 is the 205R14C for 4.86 and 21 is the 205R14C for 5.43, these two (if nla) would do for most stock use as I reckon few now run 185s, otherwise do the ones that are not available
and they are close enough for 215/75/15s, but if you use the lower value ones with the 215/75/15s the speedo is almost exactly right, but the odometer is under reading by 5% or more so it looks like the fuel economy is down when doing your sums

or do the maths and work out the best choice for 215/75/15 with z=8 for 4.86 and 5.43

The few 4.57s were only ever supplied on 185s or 205/70/14s and had to make do with the 8/18 that was used in the 4.86/185 vans so was always a compromise, but there are very few of them out there, there seem to be few 5.83s and the 16s were on such a range of tyres and mix of final drives but are all rare it's not worth worrying about them as long as brickie has one z=9 drive available

whatever you run you can either have overread speedo 10% and bob on odometer or correct speedo and underread odometer, now that most ppeps have sat nav the speedo accuracy may not be the most important consideration

what do people think ? especially our cousins over the pond ? maybe worth dropping Derek Drew a message
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 18 May 2016, 18:41

Ok so "die acht-zahn" ratios are the priority ;) The 7 & 9's will follow in due course, unless there is one in particular that favours 215/75/15

Aidan for clarity/sanity could you please to a Ctrl-V on the table of numbers I have re-ordered below and note alongside as to their application?

I would say optimize for odo accuracy as per OEM, no-one ever got nicked for having an over-reading speedometer.

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Last edited by silverbullet on 18 May 2016, 19:06, edited 1 time in total.
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 18 May 2016, 18:55

Recap:

1600 ÷ (diff ratio x wheel revs per mile) = W

Compare W to numbers below for best match

e.g. as before 185/82 R14, 4.86 diffs, 800 turns per mile

1600 ÷ (4.86 x 800) = 0.411 theoretical, in practice its the white gear 8/19 I found in my front diff, no reason to doubt it.

(But I dont know if it originally came with 6" rims. White sticker is gone so no M-codes and I dont have a birth cert.)

Or... same diffs but with part worn 215/75/15 AT2: 680mm rolling diameter = 748 wheel revs/mile

1600 ÷ (4.86 × 748) = 0.4396

E D I T So should really be running an 8/18 for the bigger tyres.

(Speedo drive ratios re-ordered by numerical size)

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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Last edited by silverbullet on 28 Jun 2017, 08:49, edited 5 times in total.
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Re: Speedo drive gears and ratio calcs for tyres

Postby bene » 18 May 2016, 20:42

Cripes I feel like I'm going to have to read this 100times to get my head round it...
My speedo recently stopped working, I assume the drive gear in the front diff has worn.
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Re: Speedo drive gears and ratio calcs for tyres

Postby silverbullet » 19 May 2016, 18:51

Made a couple of corrections to my last post above

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