Why do police cars have certified speedometers, while civilian cars do not?
In: Engineering
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
Latest Answers