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Speedometer
Accuracy
If
a speedometer is not accurate, it could
display a lower speed than the vehicle
is actually travelling at, thus causing
drivers to believe they are not
speeding when in fact they are. This
effect can occur if a speedometer
under-reads true speed.
- A
speedo over-reads if it displays
100kmh when the vehicle's actual
speed is 90kmh.
- A
speedo under-reads if it displays
100kmh when the vehicle's actual
speed is 110kmh.
The
actual speed can be easily determined
these days using GPS (Global
Positioning System). I use a meridian
GPS unit which can display speed to the
first decimal place. Motorists should
have their speedos calibrated to ensure
it is as accurate as is reasonably
possible. Up until July 2006 the
Australian Design Rules required new
cars to have speedos that are accurate
to within 10% of actual speed. The
current Rules disallow under-reading,
and permit over-reading by up to 4kmh +
10%.
Speedometer
Errors
Several factors can affect the
accuracy of speedometers:
Worn
tyres: As the rubber on the tyre
wears, the tyre diameter becomes
smaller. This will make the wheel
travel a shorter distance per
revolution. Your speedometre might
think one revolution is 170cm, but due
to tyre wear it is in fact 167cm. So a
car running at the same RPM will
actually travel less distance. Instead
of covering 100km in one hour, it goes
only 99km. As the rubber wears, the
vehicle's actual speed drops relative
to the displayed speed. Because speedos
should always be calibrated when tyres
are new, not when they are worn,
driving on worn tyres will mean your
speedo over-reads. Actual speeds will
always decrease relative to a fixed
displayed speed as the tyres
wear.
Tyre
Pressure: If you decrease the tyre
pressure, the diameter of your wheel
decreases and the wheel will travel a
shorter distance per revolution. This
will decrease the actual speed of your
vehicle in comparison with the
displayed speed. If you drive with low
pressure in your tyres you reduce the
risk of unknowingly driving over the
speed limit. Because speedos should
always be calibrated when tyres are
fully inflated, driving on low pressure
tyres cannot cause your speedo to
under-read. However, heat build up or
over inflation may cause the tyres to
balloon slightly which can increase
their diameter and could theoretically
cause under-readings if there were no
other factors offsetting this
error.
Rim
size: If you change the diameter of
your wheels you will affect the
accuracy of the speedo. Changing from
16 inch wheels to 17inch wheels does
not necessarily change the total
diameter of the wheel. It depends a lot
on the type of tyre that is used.
Changing to a wider diameter rim will
usually result in a larger diameter
wheel. A larger wheel causes your car
to travel further with each revolution.
If the speedo is not recalibrated it
may under-read, which will mean your
actual speed might be greater than that
displayed on your speedo.
Differential
ratios: Changing the gear sizes in
your vehicle's differential or gear box
can affect the accuracy of the
speedometer, resulting in either
over-reading or under-reading depending
on what the change was.
Vehicle
load: When you load your car the
tyres carry more weight, which can
cause them to depress slightly and have
a smaller diameter. This can cause an
over-reading. It can never cause
under-reading. It can not cause you to
exceed the speed limit unknowingly.
Just as you calibrate your bathroom
scales when there is no weight onboard,
so too are car speedos calibrated when
there is no extra load on the
tyres.
Speedo
Displays: Many modern speedos
display in increments of 5 kmh and do
not easily display speeds down to 1kmh
accuracy. The needle mechanism has its
own accuracy limitations. Some old
vehicles will have faulty speedos or
they have fallen out of calibration, or
the needle may wag up and down slightly
making it difficult to determine
exactly what speed is being
displayed.
Speedo
Needles: The size and shape of
needles can make it difficult to tell
exactly what speed is displayed. Given
the variables mentioned above, the
observable displayed speed is a
reasonably accurate, and usually
conservative, estimate of actual
speed.
These potential errors have been
widely
known for decades.
Australian
Design Rules (ADR)
When
vehicle manufacturers install a speedo
at the factory, the speedo has been
calibrated using data that assumes the
vehicle is fitted with new standard
tyres inflated to full pressure, on
standard rims, and usually with the
tyre circumference measured before the
wheel is put on a vehicle or when the
vehicle is on a hoist (no load on
tyre). When they test the car for
compliance with the ADR, the speedo
will invariably produce an over-read
result.
The
current Australian
Design Rules
require speedos to be calibrated when
the vehicle is unladen, fitted with
normal tyres inflated to full pressure
with an allowance for tyre
heating:
- "Unladen
vehicle" means the vehicle in
running order, complete with fuel,
coolant, lubricant, tools and a
spare wheel (if provided as standard
equipment by the vehicle
manufacturer), carrying a driver
weighing 75 kg, but no driver's
mate, optional accessories or
load.
- "Tyres
normally fitted" means the type or
types of tyre provided by the
manufacturer on the vehicle type in
question; snow tyres shall not be
regarded as tyres normally
fitted;
- "Normal
running pressure" means the cold
inflation pressure specified by the
vehicle manufacturer increased by
0.2 bar.
-
The
current ADR prohibits any
under-reading:
- "5.3.
The speed indicated shall not be
less than the true speed of the
vehicle. At the test speeds
specified in paragraph 5.2.5. above,
there shall be the following
relationship between the speed
displayed (V1 ) and the true speed
(V2).
- 0
² (V1 - V2) ² 0.1 V2 + 4
km/h."
This
formula means that the vehicle's actual
(true) speed must not be greater than
the displayed speed. The displayed
speed is permitted to be greater than
actual speed by up to 4kmh plus 10% of
actual speed. So that means that if
your car's actual speed is 100kmh, the
displayed speed is permitted to be
anywhere between 100kmh and
114kmh.
Prior
to July 2006 the ADR allowed ±10%.
This means that new cars sold prior to
1 July 2006 could comply with the ADR
even if the speedo under-read by 10%.
Despite this being theoretically
possible, due to the testing procedures
and the reasons stated below it is
unlikely to ever occur in practice.
The
reason the ADR are relevent to the
debate is that some people argue that
if the ADR allows cars to have speedos
that under-read by up to 10%, then
surely our road laws must also allow
10% tolerance. Therefore, the argument
goes, we should not be fined if we
travel at 110kmh in a 100kmh zone. Such
a situation would be quiet generous to
motorists and would make it impossible
for anyone to allege that they were
mislead by their inaccurate
speedo.
One
problem with this argument is that it
ignores reality. It assumes that
speedos are under-reading by 10% when
the truth is most speedos over-read by
about 3%. So in effect, it is allowing
the majority of motorist the
opportunity to drive at 10% above the
speed limit. If the speed limits are
there for a reason, then they would all
need to be reduced by 10% to counter
the effect of motorists increasing
their speed above it.
If
a motorist has a speedo that
under-reads by any amount, then that
person may have a defence of "honest
and reasonable belief" in a state of
facts which if true would make their
conduct innocent. (See Proudman
v. Dayman).
A Proudman v. Dayman defence is not
dependant on the existence of the
Australian Design Rule specification. A
court is concerned with what your
speedo actually displays, not what the
ADR says it could display. If your
speedo under-reads and there is no ADR
speedo specification in existence, your
defence would be the same. The 10%
tolerance specified in the ADR is
irrelevant to any defence at law. Any
driver who relies on the old ADR for
their belief that their speedo was
accurate would probably fail because
the old ADR does not require a speedo
to be accurate. It allowed it to be out
by up to 10%. And if you want to argue
that the ADR is a "law" which applies
to the State of Victoria, then you also
need to argue that ignorance of the law
is no excuse, and therefore you are
deemed to have known that your speedo
was up to 10% under-reading and you had
no basis for assuming it was correctly
displaying your speed at the time of
the alleged offence.
For
most of the 20th century there was no
requirement that cars be fitted with
speedos, seat belts and many other
items we all take for granted these
days.
Relevance
of speedo errors in
court
It
is sometimes suggested that the
inaccuracy of speedometers could be
used in court as a defence to a
speeding charge. The basis is that the
driver will allege that he or she was
mislead by the incorrect speedo.
Clearly an over-reading can not be
relevant in any defence, because then
the driver would be travelling slower
than the displayed speed. You must have
an under-reading if you want to allege
that you exceeded the speed limit
because you were mislead by your
speedo.
Motor
traffic offences are strict liability
offences. That means the prosecution
does not need to prove that the driver
had knowledge that he or she was
committing an offence. It is no defence
for a driver to say simply "I did not
realise I was speeding".
If
your car does in fact under-read, then
you might have a defence in court. This
is particularly so since the new ADR
came out. The new rules require new
cars never to under-read, so if yours
does then it fails to comply with the
new ADR. This will support a defence of
honest and reasonable
belief.
The
defence of "honest and reasonable
belief" is available in most traffic
cases, and arguably this includes
speeding cases although in Victoria
that has not always been accepted by
the courts. To win using this defence
it is necessary for the driver to show
that they had an honest belief that
they were driving lawfully at the time,
and that it was reasonable for them to
hold that belief. It is not an easy
defence to run.
If
you modify your vehicle by changing
wheel sizes or gear ratios, then it is
reasonable to expect that the speedo
will become inaccurate and may
under-read true speed. Any court will
say that a driver who chooses to drive
without checking whether the
modifications have affected their
speedo, or without having the speedo
re-calibrated is reckless as to the
accuracy of their speedo. In those
circumstances the driver should not
expect the court to accept that it was
reasonable for the driver to form a
belief that he or she was driving
within the speed limit based on the
display of an inaccurate
speedo.
The
usual wear and tear factors outlined
above all work to increase the
displayed speed (making it display a
speed greater than actual speed), and
therefore can not form the basis of an
honest and reasonable belief defence.
There is no defence available unless
the speedo has erroneously displayed a
speed lower than actual speed. i.e. the
speedo displays 100kmh when your actual
speed was 115kmh. Tyre wear, carrying
heavy loads and low tyre pressure all
result in over-readings and thus defeat
the argument.
In
order to obtain any significant
under-reading in your speedometer you
will need to modify your vehicle.
Fitting tyres with thicker tread or
over inflating will barely account for
the 1% - 2% deduction already built
into speedos at factory. If you are
driving a modified vehicle no
magistrate is going to accept that it
was reasonable to assume the speedo
would remain accurate.
If
you have purchased a second-hand car,
it may be possible that you did not
know that any modifications had been
made. It is still a hard task to
convince a magistrate that it was
reasonable for you to trust that the
speedo was accurate on your second hand
car, particularly when they are not
calibrated as part of the RWC testing.
If you get your speedo checked and find
it is under-reading, then you no longer
have any basis for trusting that it is
accurate.
If
you want to allege that it is too
difficult to read your speedo down to 1
kmh (because of the graduations on the
display, or the size of the needle),
then the court will expect you to err
on the side of caution when driving. In
any case, all that is required is for a
speedo to indicate if you are lover or
under 60kmh or 80kmh, for example.
There is no requirement, either legal
or practical, for a speedo to
accurately display a speed of 87kmh,
because we do not have any 87kmh zones
in Australia. The speedo needs to
indicate if you are over the limit. So
claiming your needle or display makes
it hard to determine if your speed was
102kmh or 104kmh is unlikely to get you
anywhere. A court will expect you to
keep the middle of the needle below the
100kmh graduation mark when in 100kmh
zones, and that a person who drives
over the limit because they could not
clearly read the exact speed must have
been reckless as to whether or not they
were committing an offence. This is not
a defence. The court expects drivers to
make sure they are driving within the
law.
How
to defend a speeding charge based on an
incorrect speedo
Prior
to July 2006 the Australian Design
Rules required speedos on new cars to
be accurate to within 10%. It is lawful
for a car have a speedo which
under-reads by 10%. Speedos are not
covered by Victoria's roadworthy laws
and are not checked as part of
roadworthy tests. If you are booked for
doing 110kmh in a 100kmh zone, and you
then get your speedo checked to find
that it is under-reading, then you
might have a good argument under the
Proudman v. Dayman defence. It is a
defence to most strict liability
offences for the defendant to show that
he or she had an honest and reasonable
belief in a state of facts which if
true would mean that no offence would
have been committed, and if this is
shown, the prosecution then have to
prove beyond reasonable doubt that the
defendant did not hold an honest and
reasonable belief. On the other hand
there is line of authority in Victoria
that suggests that speeding offences
are absolute strict liabilty offences
for which a defence of honest and
reasonable mistake does not exist.
To
rely on the Proudman v. Dayman defence
you will need the following: a
relatively new car (factory
configuration, not worn out), an
engineer's report that shows how much
the speedo under-reads, the opinion of
an expert to say that at the time when
the vehicle was being driven at the
speed detected by the police, the
vehicle's speedo displayed a speed that
was within the speed limit, and
evidence from the driver to say that he
had an honest and reasonable belief
that at the time of driving he believed
his speedo was accurate, that it
displayed a speed within the speed
limit at the time of the offence and
that he relied on his speedo to
determine what speed he was driving
at.
A
court is unlikely to accept this
defence if your actual speed was so
high that even an inaccurate speedo
would have displayed an illegal speed,
or if you did not have good reason to
trust your speedo was accurate, or if
you can not prove your speedo is in
fact under-reading.
Before
you embark on court proceedings in a
case like this, you might wish to try
writing to the Penalty Review Board at
Civic Compliance, care of the address
on your infringement notice, enclosing
a copy of the engineers report and
calibration test results, and ask them
to withdraw the notice.
Police
allowances
Victoria
Police guidelines require a deduction
of 2kmh or 3kmh from the speed reading
displayed by prescribed speed measuring
devices, depending which device was
used. This is to avoid contested cases
where the only argument in court is in
respect of the ability of the police
device accurately to measure speed down
to the last 1kmh. The 2 or 3kmh
deduction is to take into account
potential inaccuracies in the police
speed measuring equipment. i.e. the
laser or radar device might be 1%
over-reading. The 2kmh deduction
allowed for by the police is not
intended to provide any allowance for
potential inaccuracies in motorists'
speedos, although they will allege that
it has that effect.
An
unfair system
The above is an analysis of the law
as it is at the moment. Debate may rage
about whether police guidelines, or the
law, should give greater allowances to
motorists. There is nothing wrong with
having speed limits on Victoria's
roads, and nothing wrong with enforcing
those limits. A line needs to drawn
somewhere. The real issue is with
penalties. Speeding fines can be easily
incurred from a moments lack of
concentration on speed signs, or on the
speedo. The harsh consequences of
licence loss from excess demerit points
should not be inflicted on people who
are detected driving 4kmh over the
speed limit. In NSW, drivers get 3
points for driving 3kmh over the limit!
Do that four times in three years and
you lose your licence. A much fairer
solution would be to issue small fines
(up to $50) with no demerit points for
speeds up to 5kmh over the speed limit.
1 demerit point is fair for 6kmh to
15kmh over the limit, 3 points for
anything over 15kmh above the limit. In
Victoria drivers more than 25kmh over
the limit get mandatory licence loss.
But why do they have to suffer up to 8
demerit points as well? Demerit points
should only be recorded against people
who are paying a fine and avoiding
immediate licence loss. At present in
Victoria, drivers travelling 35kmh over
the limit get 6 months licence loss
plus 6 demerit points, which means they
risk suffering suspension twice for the
same offence. This is clearly wrong.
Drink driving laws give drivers demerit
points only if they keep their licence.
If they lose their licence, they get no
points because they are already
suffering a licence loss punishment and
should not have to do it twice.
Mandatory licence loss laws in Victoria
leave the courts with no discretion to
apply a more reasonable and fairer
penalty to suit the circumstances of an
offence or of the offender. Perhaps the
courts should have more discretion in
licence suspension and cancellation
cases so that the usual sentencing
principles can be applied in those
cases.
Speedo
Accuracy, faulty speedo, speedo
inaccuracy, inaccuracies, speedo
errors, speedo margins or error, wrong
speedo reading, erroneous speedo,
defective speedo, mph speedo, broken
speedo, speedo misreading, speedo false
reading. speedometre
Related
Pages:
Getting
Legal Advice
Fines
Demerit
Points
Speeding
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