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Old July 31st 10, 10:45 AM posted to uk.railway,uk.transport.london
Adrian Adrian is offline
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First recorded activity at LondonBanter: Oct 2004
Posts: 947
Default 'Ending' "the war on the motorist"

Bruce gurgled happily, sounding much like they
were saying:

The margin (maybe not so much nowadays) is necessary to allow for tyre
wear (and IIRC tyre type on some vehicles) as well as the capabilities
of a mechanical speedo; the normal consequence of tyre wear is that the
indicated speed will be progressively too high so to avoid
underindication the average speedo will probably already be over-reading
from new.


The legal requirement is that a speedometer measures road speed with a
tolerance of +10%, -0%.


Actually, it probably isn't.

It's difficult to be sure, since the Construction & Use regs aren't on
the web. The nearest that is simple to find is the requirements for the
IVA test - which are definitely nowhere near as simple as that. There's a
table of allowable readings against accurate speed.

0 under-read is true, though.

Mind you, I'd love to know what sort of tyres are being used to affect
calibration by 10% as they wear... Something like a total of 6mm
variation due to tread wear on a typical overall tyre radius of about
320mm?