Why do police cars have certified speedometers, while civilian cars do not?

Question

Why do police cars have certified speedometers, while civilian cars do not?

In: Engineering

Answer 1

Because to be certified you have to use a specific diameter of car tire.

That’s the actual reason normal Speedo’s show higher than real speed: so they won’t ever show lower than real speed, when you put on larger tires.

Additionally the tires can’t be ran bald when certified, because that also changes their diameter.

And for obvious reason the car uses the spinning of tires to determine speed. But to be fully correct it needs to know the exact diameter of the car.

Since you never need accurate speeds, you only need to know when your car says 70, that you aren’t in reality driving 75, there’s no reason to incur the extra costs and limitations