TACHO ACCURACY IS DOUBTED
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Qread in a Commercial 1/4.1.Motor article that a tachograph could be up to 20% inaccurate at the end of a two-year period. If the tachograph record could be used as evidence of speeding, as the article said, surely some provision must be made for the tachograph to be checked for accuracy before a speeding charge was brought?
When I took a loaded vehicle to have its tachograph calibrated the centre refused to do it on the grounds that the weight affected the drive-axle tyres in such a way as to make the results inaccurate. There is also an argument that because the difference in thickness of tread between a new tyre and an old one can be up to 32mm, the accuracy of the tachograph's speed and distance recording will vary according to the wear (m the tyres.
Can you comment on these points?
AAt present it is unlikely — though not impossible — that a driver would be prosecuted on the evidence of a speed trace on a tachograph record sheet. The main problem for a prosecution would be fixing the location of the offence.
The prosecution would not have to prove that the instrument was accurate because, since the High Court case of Nicholas vs Penny [1950] 2 KB 466 — which concerned a speedometer — a court will presume that an instrument was working correctly at the material time unless there is evidence to the contrary.
The 20% error you mention was probably an isolated instance and could have been due to a mechanical failure or interference.
EC Regulations 3821/85 stipulates that when in use on a vehicle the maximum tolerance of a tachograph speed trace should be within 6kmth (3mph) of the real speed and the distance recording must be within 4% of the real distance that has been covered.
Even tighter tolerances are specified for pre-installation bench tests and on installation.
It is true that a tyre's rolling radius will be affected by the weight of a load and the amount of tread on the tyre, but the effects on the accuracy of the speedometer will be minimal.
Strong head and tail winds have a greater effect.
No speed measuring device on a vehicle can be 100% accurate all the time, but it can be sufficiently accurate for normal day-to-day purposes.
In any case, prosecutions are not brought against drivers for marginal contraventions of speed limits.