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Speedometer readings

how are the speed positions determined on speedometers? Is the speed listed in the top center done by the normal average speed limit, or simply 1/2 of a speed total listed at the right?

There’s no rhyme or reason to it. The government and insurance companies want the gauge to read high on the scale at moderate speeds. The marketing folks want the gauge to go up to high numbers, even beyond what the car could reach. After a tug of war we get something in the middle.

It changes with the times. Remember in the 70s when GM has all their speedometers only go up to 85mph? Even a loaded Corvette only read that high. Whether GM tried to project a safety image or an economy image, we’ll never know. It just looked stupid to me when a VW Rabbit had its speedo go up to 100mph and my 6 cylinder Mercury Comet read 120 mph (optimistic).

These are precision instruments tested and required by the government to be accurate to about 10%. You could be doing 66 in a 60 zone with non design tire size. If you really trick the vehicle out you could get worse than 10%. If you follow the recommended tires then your error is closer to 3% but dont tell that to a state trooper. I stick to the mandated maximum error by dot standard when ever I talk to the trooper. Remember if you say how fast you were going you have admitted to a violation. Just say your girlfriend was talking your ear off and you dont know how fast you were going.