Dan Holdsworth had it:

On Fri, 31 Mar 2006 12:15:05 +0100, Alex Heney

was popularly supposed to have said:

Speedometers, by law, may not read low.

They can be out by up to 10% high, but not low at all.

If you think about it, a speedo works by measuring the angular rotation
of the wheels of the vehicle, and assumes that the tyres are a certain
diameter in order to work out the speed.

Of necessity, that introduces inaccuracies. Over-inflation of tyres
makes the effective wheel size bigger, and the tread depth also may vary
from a minimum of 1.2 mm at replacement time to as much as 10 mm when
new.

I believe that police cars fitted with VASCAR have to have it
recalibrated as the tyres wear town.

--
David
=====
replace usenet with the

Dan Holdsworth wrote:
||
|| If you replace the tyres of a car with non-standard ones, beware
|| because you may make the effective wheel diameter bigger and thus
|| make your speedo under-read. This particularly applies to drivers of
|| old Landrovers, which when new were fitted with tyres which measured
|| in Imperial units in a way that nobody uses any more, so getting the
|| exact, to-spec tyre for these is pretty much impossible.

A quite useful calculator he

http://www.clubpolo.co.uk/articles/005.htm

--
Rob

On Fri, 31 Mar 2006 20:10:04 +0100, Dan Holdsworth
wrote:

Of necessity, that introduces inaccuracies. Over-inflation of tyres
makes the effective wheel size bigger, and the tread depth also may vary
from a minimum of 1.2 mm at replacement time to as much as 10 mm when
new.

Tread depth makes a difference, but not inflation. A flat tyre will
move almost the same distance in one revolution as an inflated tyre.
This is because unlike a balloon, inflation does not stretch a car
tyre appeciably, so the circumference remains the same (though
distorted from a true circle). Perhaps think of the track of a tank.
The difference in rolling circumference to what the vertical radius
would calculate to is taken up by continuous flexing of the sidewall
and tread as the vehicle rolls, which is why a flat tyre gets very
hot.

--
Cynic

"Alan Holmes" wrote in message
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"Nick" wrote in message
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"Alan Holmes" wrote in message
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"Dr Zoidberg" wrote in message
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Alan Holmes wrote:
"Dr Zoidberg" wrote in message
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another MickG wrote:
Scenario.

You pull off a roundabout, and accelerate away from the roundabout.
You accelerate up to 38 mph. You then realise your mistake and slow
down to 30 mph,( you are in a 30mph zone).

But you got zapped before you slowed down. So, concequently you are
sent a fine.

As is legally correct.
This is some way above the speed limit and while it may be the sort
of speeds that I do regularly I'm well aware of the risks.

Opinion.

I would say that accidently speeding and slowing doen to the speed
limit when you realise your mistake is responsible, and not at all
irresponsible.

Ok , then accellerating above the speed limit was careless.
You can have three points for due care and attention instead.

Assuming that the allegation was absolutely true!

The driver admits that they went too fast then slowed down.

But how can he be absolutely sure that he was going too fast, when
speedometers are not all that accurate anyway?


Either way, I wouldn't have thought that the fact that a speedometer is
not
working properly would be an excuse for speeding.

But, if it had been overreading by a significant amount he would not have
been exceeding the speeed limit!


You know that and I know that, but would he know that!

The driver wouldn't know either way and therefore unless he knew the amount
by which it was out he would be accepting the risk that he was over the
limit every time his speedometer was showing that he was over the limit -
whereas, of course, it is quite true that in fact he might not be over the
limit.

Fortunately the instrument that made the reading would have to be a lot more
accurate than the speedometer in the driver's car and if it could be shown
that it was calibrated incorrectly that would be grounds for an appeal or an
acquittal.

Nick

"Cynic" wrote in message
...
On Fri, 31 Mar 2006 20:10:04 +0100, Dan Holdsworth
wrote:

Of necessity, that introduces inaccuracies. Over-inflation of tyres
makes the effective wheel size bigger, and the tread depth also may vary
from a minimum of 1.2 mm at replacement time to as much as 10 mm when
new.

Tread depth makes a difference, but not inflation. A flat tyre will
move almost the same distance in one revolution as an inflated tyre.
This is because unlike a balloon, inflation does not stretch a car
tyre appeciably, so the circumference remains the same (though
distorted from a true circle). Perhaps think of the track of a tank.
The difference in rolling circumference to what the vertical radius
would calculate to is taken up by continuous flexing of the sidewall
and tread as the vehicle rolls, which is why a flat tyre gets very
hot.

Maybe some cars can take tyres of a larger circumference than intended by
the manufacturer - in which case, no doubt, the car would be illegal as the
speedometer would be incorrectly calibrated.

Nick

(Nick) wrote in
:


"Alan Holmes" wrote in message
...

"Nick" wrote in message
...

"Alan Holmes" wrote in message
...

"Dr Zoidberg" wrote in
message ...
Alan Holmes wrote:
"Dr Zoidberg" wrote in message
...
another MickG wrote:
Scenario.

You pull off a roundabout, and accelerate away from the
roundabout. You accelerate up to 38 mph. You then realise your
mistake and slow down to 30 mph,( you are in a 30mph zone).

But you got zapped before you slowed down. So, concequently you
are sent a fine.

As is legally correct.
This is some way above the speed limit and while it may be the
sort of speeds that I do regularly I'm well aware of the risks.

Opinion.

I would say that accidently speeding and slowing doen to the
speed limit when you realise your mistake is responsible, and
not at all irresponsible.

Ok , then accellerating above the speed limit was careless.
You can have three points for due care and attention instead.

Assuming that the allegation was absolutely true!

The driver admits that they went too fast then slowed down.

But how can he be absolutely sure that he was going too fast, when
speedometers are not all that accurate anyway?


Either way, I wouldn't have thought that the fact that a speedometer
is not
working properly would be an excuse for speeding.

But, if it had been overreading by a significant amount he would not
have been exceeding the speeed limit!


You know that and I know that, but would he know that!

The driver wouldn't know either way and therefore unless he knew the
amount by which it was out he would be accepting the risk that he was
over the limit every time his speedometer was showing that he was over
the limit - whereas, of course, it is quite true that in fact he might
not be over the limit.

Fortunately the instrument that made the reading would have to be a lot
more accurate than the speedometer in the driver's car and if it could
be shown that it was calibrated incorrectly that would be grounds for an
appeal or an acquittal.

Nick





Thank you everyone for discussing this. It's been very interesting, with
some very thoughtful points, and some not so thoughtfull points.

Leaving asside the accuracy of speedometers. Can I just add a point to the
discussion, regarding speedos.

When your driving you can't look at the speedo all the time, hence you
could very easily creep over the limit, until the next time you look at he
speedo. This is what happened to my Dad.

My Dad considered keeping his eye on the road more important that looking
at the speedo, but when he did look at the speedo, he noticed that he was
over the limit and slowed down. But in that short space of time, he was
caught.

Tell me, how can speed cameras take such situations into account. I would
say that it's impossible. In just a couple of weeks research, I have
learned that there are massive divides between police forces thoughout the
country about the use of camera's. Durham use a bare minimum, mainly
because of the points I have raised, they believe they catch the wrong
people. Durham believe that the drivers caught on camera are about 90%
people just making short mistakes, cameras do not catch dangerous drivers.
Durham prefers to rely on patrols and traffic calming measures. They claim
a very high success rate.

On the other hand, Birmingham have a different approach. They say that
cameras, are catching the offenders that would not normally be caught by
patrols. As far as Birmingham are concerned you were over the speed limit,
even if you corrected your speed, they thing the camera never lies.
Birmingham claim that hey are a success because of the ammount of speeders
they catch.

Leaving aside my Dad. I was actually trying to raise a discussion about
how cameras are used, and if they are really actually "working". i.e. do
people just slow down between the camera's, and speed up when they are not
there ? Are they being used to fine/punish people for making small
mistakes ? Or do they actually make the roads a safer place ???????