'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?
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