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%.

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?

"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?

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.

"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.

On Sat, 31 Jul 2010 12:36:57 +0100, "Recliner"
wrote:
"Bruce" wrote in message

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.


I agree - I have had several cars over the last couple of years, and
all have had speedos that showed 5% over.


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.


That's car manufacturers for you. Extras and higher trim levels are
where they make their money. Mercedes used to be the worst for this -
they offered a very basic car (often referred to as "taxi trim") at an
apparently low price, then you had to pay through the nose for
necessary features. Of course they could claim that Mercs had the
lowest depreciation in the business, but that is because the used
values were related to an artificially low list price ...

On the other hand, my partner's Skoda Octavia has tyre pressure
monitoring as standard.


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.


That sounds more like 10% over rather than 5%. Please explain?

On 27/07/2010 13:49, amogles wrote:
On 27 Jul., 13:35, wrote:


I believe that they make a small net loss (ie, raise less than they
cost), but that's probably not the real reason for withdrawing funding
for them.

If this is the case, then I wonder why so many people have got away
with claiming that speed cameras were just a stealth tax, and more
importantly, why these claims were never challenged by those who new
better. I've never heard of a tax that costs more to collect than it's
actually worth.

This is why the dog licence was abolished - it was costing more to
administrate it brought in. Perhaps also the radio licence as well - not
quite so sure about that.

Its quite possible for things like speed cameras to cost a lot but also
bring in a lot of money. So motorists who claim them to be a cash cow
may be right, but people who say they will save lots of money by
abolishing them may also be right.

--
John Wright

Blasphemy - a victimless crime.

"Bruce" wrote in message

On Sat, 31 Jul 2010 12:36:57 +0100, "Recliner"
wrote:
"Bruce" wrote in message


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.


That sounds more like 10% over rather than 5%. Please explain?

Yes, the adaptive cruise control needs to be set approximately 10% over
the targeted true speed for some reason. The analogue speedo then shows
a speed about 5% over the true speed. It was a bit disconcerting at
first, but I soon got used to it, and haven't bothered to raise it with
the garage as I doubt that they'd know what I was talking about.

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.

--
Graeme Wall

This address not read, substitute trains for rail
Transport Miscellany at www.greywall.demon.co.uk/rail
Photo galleries at http://graeme-wall.fotopic.net/

In message
john wright wrote:

On 27/07/2010 13:49, amogles wrote:
On 27 Jul., 13:35, wrote:


I believe that they make a small net loss (ie, raise less than they
cost), but that's probably not the real reason for withdrawing funding
for them.

If this is the case, then I wonder why so many people have got away
with claiming that speed cameras were just a stealth tax, and more
importantly, why these claims were never challenged by those who new
better. I've never heard of a tax that costs more to collect than it's
actually worth.

This is why the dog licence was abolished - it was costing more to
administrate it brought in. Perhaps also the radio licence as well - not
quite so sure about that.

The radio licence coevered cost but became almost impossible to police with
the proliferation of portable receivers.


Its quite possible for things like speed cameras to cost a lot but also
bring in a lot of money. So motorists who claim them to be a cash cow
may be right, but people who say they will save lots of money by
abolishing them may also be right.


Diffeent groups of people involved in the payment and collection.

--
Graeme Wall

This address not read, substitute trains for rail
Transport Miscellany at www.greywall.demon.co.uk/rail
Photo galleries at http://graeme-wall.fotopic.net/