On 08/06/16 12:59, NY wrote:
"Tim Streater" wrote in message
.. .
Ha! Not in Morse's MK2 Jag, though, eh, where the speedo needle
fluctuated wildly.

There's some in-car footage in one of the episodes of The Sweeney where
a baddy was driving a S-Type Jag and the speedo needle is all over the
place. Not sure whether that was normal or whether the speedo was f*cked
because they tended to use cars that were about to be scrapped for any
car chase sequences.

The old eddy current speedos were very prone to get bits of magnetic
material between the magnet and the eddy plate.


BTW Wiki reveals this:
------------8
United Kingdom
A speedometer showing mph and km/h along with an odometer and a separate
"trip" odometer (both showing distance traveled in miles).

The amended Road Vehicles (Construction and Use) Regulations 1986
permits the use of speedometers that meet either the requirements of EC
Council Directive 75/443 (as amended by Directive 97/39) or UNECE
Regulation 39.[14]

The Motor Vehicles (Approval) Regulations 2001[15] permits single
vehicles to be approved. As with the UNECE regulation and the EC
Directives, the speedometer must *never show an indicated speed less
than the actual speed*. However it differs slightly from them in
specifying that for all actual speeds between 25 mph and 70 mph (or the
vehicles' maximum speed if it is lower than this), the indicated speed
must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the
speedometer must not show more than 61.25 mph or less than 50 mph.
8------------------------------------------


So speedos are DELIBERATELY designed such that no combination of tyres
pressures and loads will ever result in them under reading, and they can
in fact overread by MASSIVE amounts and still be 'street legal'

So the next time you are sitting in the fast lane at what you think is
70, bear in mind you might be doing no more than 58mph...

Irrespective of how that is arrived at, unless you are using GPS or
doppler you WILL be counting rotations of prop shaft or wheel, so the
tyre loading/pressure/make/wear issues are all still in the mix.




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