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Can we assume that our satnavs in general, whether M & M or others, measure speed accurately with reference to the satellites, whereas speedos tend to measure about 10% high? In speed limited road works (e.g. between J5 & J6 on M25), setting the cruise control at 50mph from the satnav, takes me sailing gently passed most traffic which presumably is going by their speedometers.

This is not confined to our up!, but the same thing occurs when using TomTom in other cars - strange. Perhaps one of the techies out there can throw some light on this
 

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When i use Cruise Control on my C4 and set it to 50mph i sail past people yet my TomTom reckons I'm only doing 47-48mph. Its amazing how many people can't do actual 50mph.
 

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Si599 said:
When i use Cruise Control on my C4 and set it to 50mph i sail past people yet my TomTom reckons I'm only doing 47-48mph. Its amazing how many people can't do actual 50mph.
Not really. A 50 mile per hour speed limit is the maximum speed. There is no requirement to drive at that speed but at a speed suitable for the conditions which does not exceed the limit.
 

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Speed displayed is bound to be related to wheel rotation - and distance actually covered will also depend on tread depth - so no speedo is ever going to be 100% accurate all the time.
 

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A lot of people are so desperate not to get a speeding fine because of having to shell out and again and again for the next five years through increased insurance costs that they will drive well within the limit and i count myself amongst that group. Also, occasionally, i've noticed my speed has accidentally increased to 55mph which meant i've had to drop to 40-45mph to get my average speed down in time before the next camera.Edited by: AlanG
 

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speedos always read high in my experience, as do mpg readouts !
 

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From what I read on this a while ago, they are set 'low' by the manufacturers due to Type-approval regulations (see Wikipedia):
<ul>[*]The indicated speed must never be less than the actual speed, i.e.
it should not be possible to inadvertently speed because of an incorrect
speedometer reading.[*]The indicated speed must not be more than 110 percent of the true
speed plus 4km/h at specified test speeds. For example, at 80km/h, the
indicated speed must be no more than 92km/h.[/list]So they build in enough margin to accomodate both of these and take into account slightly different readings due to wear, ambient temperature, etc.
 

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Another contributor is wheel/tyre diameter. The Up! can be fitted with 14, 15 or 16 inch wheels which can accommodate a range of standard and plus size tyres. The speedo is calibrated to accommodate the range of options that may be legally fitted.
 
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