'Ending' "the war on the motorist"
"Adrian" wrote in message
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?
Wouldn't tyre pressure have a much bigger effect?
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