Current systems cannot know the exact speed of a car due to the mechanics involved:
A speedometer's reading is measured on the gearbox, at the output but there are a myriad of little things after that including variance in the final drive, tire size and tire pressure all of which can make the "calculated" speed and the real speed to be within a few % of each other. All of these components are also affected by wear too.
At the moment you need a GPS to conclusively measure a car's speed with >99% accuracy but GPS signal isn't available/sufficient everywhere. One could argue that we should create some sort of highly accurate local system that measures speed and we certainly can but we'd be trying to solve a 0~10% speed measurement problem: we'd get no real value from it.
In other words: a car measures how fast a specific cog is rotating, adds a few variables and estimates how fast the car is going based on that.
edit: and all of this assumes the car is going straight, calculating speed when the car is curving is more complex.
A speedometer's reading is measured on the gearbox, at the output but there are a myriad of little things after that including variance in the final drive, tire size and tire pressure all of which can make the "calculated" speed and the real speed to be within a few % of each other. All of these components are also affected by wear too.
At the moment you need a GPS to conclusively measure a car's speed with >99% accuracy but GPS signal isn't available/sufficient everywhere. One could argue that we should create some sort of highly accurate local system that measures speed and we certainly can but we'd be trying to solve a 0~10% speed measurement problem: we'd get no real value from it.
In other words: a car measures how fast a specific cog is rotating, adds a few variables and estimates how fast the car is going based on that.
edit: and all of this assumes the car is going straight, calculating speed when the car is curving is more complex.