It is usual for the speedo to be geared to the pinion or crownwheel of
the differential (thus measuring the average speed of the drive wheels).
Pickup is usually in the gearbox directly coupled to the output shaft in
RWD cars, and to a shaft geared to the diff in FWD cars (going from
memory with my Issigonis-style Minis here :) ). In later models the
pickup sensor is electronic, but still has a mechanical gearing to the
same pickoff point.
My father was a cabbie many years ago, and the law requires periodic
taximeter calibration. This was ALWAYS done with a new set of tyres -
often kept aside by some operators just for this purpose. :)
In South Australia we have a 10% accuracy requirement. This takes into
account tyre wear and speedometer mechanical inaccuracy. It appears
that, from the doppler devices I have eyaballed in passing on foot (many
years ago), that they were set to 10% of the limit +2km/h (this is
probably no longer accurate!). We have many devices capturing this sort
of thing now:
1- car mounted doppler speed cameras (always cosine corrected though)
2- red light intersection cameras (now almost all replaced with (3))
3- 'safety cameras' looking for both red light & speed violations at
intersections (2 & 3 are inductive coil delay triggered)
Re the GPS, a few years ago, a motorist was tagged at a safety camera
intersection as speeding. He happened to be route recording with the GPS
unit, and was able to show that he was in no way speeding, & the case
was dismissed. Soon after this the laws chaned so that GPS evidence was
no longer admissible in these cases...
davidt
On Thu, Jan 17, 2013 at 04:11:50PM +0000, Chris Barker wrote:
> You're right, I think, Brian.
>
> The most accurate indication of speed is probably a GPS readout.
>
> Chris
>
> On 17 Jan 2013, at 12:41, Brian Gray <bsg017@xxxxxxxxxxxxxx> wrote:
>
> > As far as I know car speedometers are actually geared to the rotation
> > rate of the wheel. So the accuracy of the speedometer will vary as the
> > tyres wear, perhaps by 3% or more over their life. In the UK I note
> > with interest that the 'doppler' speed indicators which light up at the
> > start of some speed restrictions if you are still over the limit
> > generally seem to agree with my speedo reading on newish tyres.
> > Brian
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