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Cop cars usually have a code in the trim tag that states that it was built as a 9C1 (which is the RPO code for a COPO car called a police car).
All police cars come with a speedometer that says "certified" on the face of the gauge. The 9C1 was the super sport of Impalas as it had Heavy Duty (HD) radiator so it could idle for hours outside of the donut shop. IN addition to a HD suspension because it is cheaper to build a HD car than it is to fix pot holes in the streets of the city. A HD alternator and battery to power the emergency lights and the radio in the days before LED lighting and transistors. A some time high horsepower engine could be found under the hood if it was used by the state police for traffic patrol. Otherwise it would have a six cylinder or a small two barrel V8 for inner city police work where fuel economy is more important that going over 40 mph.
You will note that the best description of a police car is from the movie the Blues Brothers
Jake asks, " ... What the hell is this?"
Elwood said, "This was a bargain. I picked it up at the Mount Prospect city police auction last spring. It's an old Mount Prospect police car. They were practically giving 'em away."
Jake says "Well thank you, pal. The day I get outta prison, my own brother picks me up in a *police* car!"
Elwood later said, "It's got a cop motor, a 440 cubic inch plant, it's got cop tires, cop suspensions, cop shocks. It's a model made before catalytic converters so it'll run good on regular gas. What do you say, is it the new Bluesmobile or what?"
[a brief thinking pause while Jake attempts to light a cigarette]
Jake says "Fix the cigarette lighter."