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Speedo inacuracy

Posted By : Matthew Holland, 25-Jul-2003, 05:55pm

The other day I praised my Herald 1200 on managing 90 on an empty (and downhill of course) stretch of the M25. I was however disappointed when I glanced at my GPS receiver which informed me I was only just aproaching 80 (77 to be precise).
I know all speedo's are always optomistic but is it normal for "our" speedos to be so inaccurate?

Cheers
Matt

Followups :

Speedo inacuracy

Posted By : John Davies, 25-Jul-2003, 11:48pm

Matt,
Your speedo's accuracy depends on more than mere old age. It was calibrated for the original diff ratio, wheel diameter and tyre profile. Changes in any of these will make the speedo less accurate. There are shops that can recalibrate them, if you stipulate the other variables.
John
- Speedo inacuracy
  
  Posted By : Colin Lindsay, 27-Jul-2003, 01:04pm
  
  I've no doubt that speedos are great in the lower speed ranges but the faster we go, the more inaccurate they get... I don't think the Herald was ever intended to get near 90!!
  - Speedo inacuracy
    
    Posted By : Ben Ferguson, 27-Jul-2003, 02:11pm
    
    Oh well, had my 13/60 up to an idicated 90, very nearly flat out, with no overdrive, so was absolutely screaming, but if it is meant to do a genuine 85, the speedo cant be that far out surely?! I'm pretty sure the engine is putting out as much power as it should.
    - Speedo inacuracy
      
      Posted By : Savvas Regas, 16-Aug-2003, 07:47pm
      
      Hi
      
      I've read somewhere (I'll try and remember where) that the original speedo is accurate to about plus or minus 9 or 10%. It follows therefore, that if your true speed is 80 MPH, your indicated speed (to use aeronautical terms) may be anywhere between 72 and 88 MPH assuming + or - 10% which is indeed a considerable range.
      
      I ahve noticed that on my speedo, it is considerably steadier when my foot is on the gas, rather than just 'running on'.
      
      Regards,
      sav