Why do police cars have certified speedometers, while civilian cars do not?
In: Engineering
Because a court can throw out a speeding ticket if the cop was relying on an uncertified speedometer as evidence to write the ticket.
because the radar in a cop car can only tell the difference between its self and a target car’s speed, not the actual speed.
If the cop car’s speed is not known with certainty, neither is the target car’s speed.
And you cant write a speeding ticket based on “well they may have been going to fast, that or my speedometer is wrong”
The speedo is calibrated, because cops need to be able to prove that, when they claim you were going ___ km/h, the evidence will support the claim.
It’s illegal for a speedo to show you are travelling under 30mph when you are doing over so manufacturer’s actually calibrate your cars to lie.
Because parts wear and people usually don’t have the correct tyre pressure in tyres with optimal tread, your car will say 30 but you’re probably doing 27 or 28.
A sat nav will give you a more accurate reading.
To answer the 2nd half of your question, having a speedometer certified is an extra cost that’s not necessary for civilian cars. Being close enough is good enough for civilians.
If the police is going to use their speedometer as evidence, then it needs to be checked periodically to make sure it’s still providing valid evidence.
You don’t use your speedometer as a legal documentation device.
Another example are scales, your bathroom scale doesn’t need to be rechecked, but a truck scale on the side of the road does.
For most people, near enough is good enough so long as you’re not speeding without knowing it so the requirements for speedometers for general use is to show no slower than you are actually going and only a bit faster. There is an international standard that specifies this as no more than 4km/hr + 10% faster than actual and no less than actual as this has so far been accepted as an acceptable error to allow for manufacturing tolerances and difference in things like tyre wear than can change the number slightly
For police it’s different as they rely on knowing exact speeds to be able to say not just that you were speeding but by exactly how much. To do this, they need to know exactly how fast they’re going or at least to a much narrower margin than the spec I referenced above.
Edit: In particular, it now matters if they are going slower than they think as they will now be measuring a greater difference in speed than there actually is. At highway speed (100km/hr) standard spec could mean showing someone going 114km/hr when they’re actually doing 100 if the both cars speedos are at opposite ends of the tolerance
In addition to all the other comments about cops needing to have an accurate speedo in order to measure other cars’ speeds: uncertified speedos tend to skew towards reading high. If your speedo says you’re doing the speed limit, chances are you’re a little under it. If you are actually speeding, the speedo will say you’re going even faster again.
If a police speed read high like this, they would overestimate the speed that they (and the car they’re following) is going, resulting in potentially issuing fines to people who were not speeding.
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
As others have said to be reliable for speed checks by the police, but I also wanted to add that civilians cars speedometer always show a higher speed than they are actually going.
Not sure about the us laws, but it might be up to 10% +3 higher what is shown, than the actual speed.
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