> Next time you meet a Garda, ask when was the last time their speed gun
> was calibrated and wait for the inevitable "huh?". You can generally
> expect a 10% error in your speedo and about the same in their
> gun. Late
> last year we had a car with a digital speedo set using cruise
> control to
> exactly 100kph (national speed limit), a handheld GPS also
> tracking the
> speed, and a (rather new) motorbike running beside for a short period.
> All read rather different speeds, with a general diff of about 10% or
Car speedometers are always calibrated to error towards
slightly optimistic values. Unless it is faulty it will
normally show something like 31 mph when actually doing 30 mph.
Generally the error margin worsens as the speed increases.
At an indicated 70 mph you may only be doing 67 mph. And
when at an indictated 140 mph you may well only be doing 135 mph.
One of the most accurate speedometers around currently is that
which is fitted to the latest Rolls Royce which was
road tested in this weeks Autocar magazine. And for every
actual speed it was indicating exactly 1 mph more (for every
reading from 0 mph up to a tested 140 mph).
The reason that the speedometers are designed to error
in that way is to reduce the possibility of law suits against
manufacturers whereby owners received speeding fines etc
believing they were under the limit. So if the speedometer
doesnt behave this way, it's defective and that makes it the owners
repsonsibility to ensure the car is legal as with lighting and
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