On Fri, 30 Jul 2010 23:51:49 +0100, Charles Ellson
wrote:

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%.