"Bruce" wrote in message

On Sat, 31 Jul 2010 11:52:24 +0100, "Recliner"
wrote:

"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?


Indeed so, and that forms the basis of tyre pressure monitoring in
many modern cars - when the rotational speed (RPM) of one wheel
exceeds that of the others by more than a predetermined percentage, an
alarm sounds and a warning light illuminates.

I repeat that the legal requirement is that a speedometer measures
road speed with a tolerance of +10%, -0%. I am told by a friend who
is a car designer that this is stated in the Construction & Use
Regulations. He says it is very out of date because speedometers can
now be made to much tighter tolerances.

Yes, whenever I've compared my speedo reading in modern cars with an
accurate GPS, the speedo is very close to 5% over. I suspect that
manufacturers always aimed at this, but modern electronic devices
deliver it accurately. Given that any car with ABS presumably can
measure wheels speeds, it's annoying that most car manufacturers charge
so much for the tyre pressure monitoring option.

Oddly enough, however, when I set the digital cruise control, it doesn't
quite agree with the more accurate analogue speedo. In other words, if I
want to do exactly 50mph through motorway road works, a speedo reading
slightly above 50mph is OK, but I need to set the digital cruise control
to something like 55mph to achieve this precisely (if the traffic is
heavy, I don't bother, and just let the adaptive cruise control keep my
speed the same as the cars I'm following).

When I reach the end of the road works, (if necessary) I change the
target speed on the cruise control to, say, 77 mph to ensure that I do
exactly 70 mph. Again, the ACC slows me down if needed.