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It is not standard practise for car makers to make speedos run 4klms,
or any other amount of klms slow. In fact efforts are made to avoid
this, as motorists can get caught for unknowingly speeding. You find
similarities in inaccuracy within makes because they frequently source their speedos from only one, or at most two suppliers. Mechanical speedos of yesteryear varied wildly[ my 1976 Chrysler Centura was
12klms slow at 100kph, my 1964 Jaguar 420 was 14klms fast at 100kph. Speedos have improved steadily, and my 2001 Astra is 4klms
fast at 100, and my 2006 Corolla is 1 klm fast. Almost all currently
produced cars are made to be either spot on, or at most 2 or 3% fast at 100kph, and for the most part they succeed. The inaccuracy of mechanical speedos is a linear progression, provided that all the parts
are working properly. As there was not a mechanical connection
between the cable and the needle, stickiness in the mechanism within
the speedo itself could cause the linearity to be lost. I was involved in
complying imported cars, and one of the most important jobs was testing, and if necessary, recalibrating their speedos, and fitting the correct
tyres so the speedos were as accurate as possible.
Sorry to ramble.
raymo
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