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!