Here in Wilmington there is a shop that calibrates speedometers (as well as repairing old or installing new "heads" as they call them). They are accepted/recommended by the Police Dept. Perhaps your community has a shop like that?

My tires on the Jag were oversized so if speedo said 70, I was really doing around 80. That shop was available if I wanted my speedo accurate with the size tires I was running, and the police were all for my speedo being accurate. I did not opt for that option and don't know what they did to recalibrate, but the shop only worked on speedometers and no other instruments.

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Nello Tare

1982 300D (266,001 mi.) looking and running great

Previously owned:
1976 450 SLC
1983 300D
1976 300D
also
1982 Jaguar XJ6 (loved, but gone)