XPost: comp.os.linux.misc   
   From: tnp@invalid.invalid   
      
   On 09/12/2025 21:01, Richard Kettlewell wrote:   
   > The Natural Philosopher  writes:   
   >> In the UK we have little 'this is your speed!' displays in green or   
   >> red depending on whether you are exceeding the limit or not.   
   >> These agree EXACTLY with my Tomtom Go  speeds, derived from GPS and   
   >> are always around 7% lower then the speedometer reads.   
   >>   
   >> On every car I have tried it on.   
   >   
   > Last time I checked my car speedo read 68mph when GPS reads 70mph, so a   
   > little under 3% off. I’ve not checked the error at lower (or higher l-)   
   > speeds, nor often enough to say how it varies with time and conditions.   
   >   
   You can calculate the effect of tyre wear quite easily.   
      
   I am sure your maths is up to that :-) :-)   
      
   Without a car tyre fitting place less easy to measure the change in   
   circumference between inflated,  deflated and overinflated.   
      
      
   > Previous cars had less accurate speedos.   
   >   
   Yes.   
      
   I think mostly they are counting sensor pulses on a wheel or final drive   
   somewhere, but IIRC its possible to build a Doppler shift radar unit .   
      
   Measuring wheel/final drive RPM is inherently inaccurate.   
      
      
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   its been subverted by the people it tried to warn you about.   
      
   Anon.   
      
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