"Graeme" wrote in message

In message
"Recliner" wrote:

"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 He says it is very out of date because speedometers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.



Interesting numbers, how old is your car? My two year old Laguna
measures a lot closer to the limits than that, 73mph indicated gives
just about 70 on the Sat Nav.

Yes, your car's speedo is the normal ~5% over, just like mine and most
other modern cars. Does it also have a digital adapative cruise control
(ACC) where you can pre-set an exact target speed, adjustable in 1 mph
increments? That's where mine is about 10% over. I don't think ACC is
standard on any cars, and is a fairly expensive option when offered.