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Old 11 May 2017, 17:21 (Ref:3732853)   #13
Dave Brand
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Join Date: Dec 2001
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Hadfield, Derbyshire (UK)
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Originally Posted by Adam43 View Post
FWIW, I find most modern cars read a consistent mph higher than the real speed, not a percentage. Most old cars tend to be a percentage, possibly because of modifications or different wheels or tyres.
Older cars had mechanical speedos, so the reading was directly related to the speed of rotation of the cable; the drive gear was slightly different to what was required to give a true speed reading. Modern speedos are electronic, so they can be programmed to read whatever the manufacturer decides.

The Chevrolet Impala we rented in the US last year had the facility to switch the speedo reading to km/h; the dial had only one set of figures . . . want to see anything above 140 km/h & you have to use the digital display.

American car speedos, in my recent experience, read true speed, as checked against GPS.
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