"thoss" wrote in message

At 13:44:35 on Tue, 12 Feb 2008 Roland Perry opined:-

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.

I have a hand-held GPS that shows my car's speedo over-reads by about 6%
or 7%. One oddity is that my cruise control allows me to set an exact
speed (digitally), which then doesn't agree with analogue speedo (even
though I assume they are both driven off the same digital source,
probably in the gear-box). My GPS shows that the analogue speedo is more
accurate than the cruise control. So, if I want to drive at exactly
70mph, I would set the cruise control at 75 or 76 mph.

One reason speedos have to over-read is that even the most precise
speedo driven from the drive train will not be exactly right -- it
depends on your tyre pressures and air temperature, as well as tyre
characteristics. If you have been driving at speed for a while, the
tyres will warm up, increasing the pressure in the tyres, and hence
their rolling radius. So, with the exact same speedo reading (and cruise
control setting), the car will actually travel a bit faster.

My car is available with 18", 19" and 20" wheels. All have tyres with
the same nominal external diameter, but the higher profile tyres fitted
to the smaller diameter rims will be less stiff than the ultra-low
profile tyres fitted to the 20" rims. The latter will therefore have a
slightly larger rolling radius, so the car will be travelling a bit
faster with the same speedo reading. The rolling radius will reduce
slightly if the car is heavily loaded.

This is all in addition to any manufacturing tolerances in the speedo
itself. That's why manufacturers typically aim at a +5% reading, so the
actual readings will be in the 0 to +10% error range.