In message , at 18:20:31 on Tue, 12 Feb
2008, thoss remarked:
My current car indicates 60mph at a true 56mph. And similarly pro-rata
at higher speeds.

I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

They are legally required to. Any error *has* to mean the speedo is
over-reading.

ISTR that the Construction and Use Regulations require speedos to be
accurate to +/-10%. I don't think you can rely on yours over-reading.

Absolutely no margin allowed for under-reading. Sources differ slightly
with regard to over-reading, but the limit seems to be +10% + 4kph (from
Hansard in 2001).

As there are apparently anything up to seven amendments *per year* to
the Construction and Use Regulations, I'll leave it to someone else to
find the exact cite.
--
Roland Perry