In “The Blues Brothers” [1980], when the scene shows the speedometer at 120 MPH, the speedometer also has the text “CERTIFIED” in the top-center, which only police vehicles at the time had.

Extra context: in the days before traffic cops had RADAR to verify how fast a speeder was going, cops had to catch-up to a suspected speeder, and record the speed once they did, hence the need for a speedometer that was “certified” as accurate, so the results would hold up in court.