"Robin" wrote in message
...
On 13/02/2019 16:54, Dave Plowman (News) wrote:
In article ,
Brian Gaff wrote:
Actually my neighbour had a Metro and boy could that thing move. The
speedo kind of just hit the end and after that it was guesswork. Only
problem with it was that it tended to be full of rust even when new.

As I said - a very optimistic speedo. Some makers today do just the same.
When making one which is within 1% accuracy should be child's play since
they are counting pulses.


I find it hard to see how a simple count of pulses can deliver 1 per cent
accuracy. The speedo *mustn't* read low. That's a statutory requirement:
"the true speed shall not exceed the indicated speed" in your no-doubt
well-thumbed copy of the The Motor Vehicles (Approval) Regulations 2001.
Wear on tyres alone can produce a difference of 2 per cent in the
circumference.

I'm surprised it's that much. Is the difference between the external
diameter of a new tyre and of a just-legal tyre as much as 2%?

I presume manufacturers design the speedo to over-read by at least 2% so it
never under-reads even with just-legal tyres. Mine seems to under read by 5
mph across the whole range from 10 - 70, compared with a GPS speedo app.