[FireBeyond]

If you are manufacturing a vehicle/speedometer in 2019 with a 10% margin of error, I'd argue that was "not fit for purpose".

I left Australia in 2006, but even before then, the government had altered the statutes on speeding from the old model - 10% margin of error, to a flat 3kph(2mph) due to "increased accuracy in manufacturing".

[rkangel]

Speedometers measure speed by counting wheel rotations and multiplying them by the circumference. There's at least a few percent inherent error as circumference is going to vary with what tyre is fitted and how worn it is.

F1 cars, and I believe now high end sports cars use a differebt system that's basically how an optical mouse works. They image the road underneath the car and measure it moving past. This gives turning in addition. If you watch an F1 night race you can see a glow on the ground under the nose - that's the illuminator for the sensor.

[moftz]

There are also Ka-band Doppler radar systems used for sensing speed on the vehicle (exactly like how a radar gun works). You mount it on the vehicle to point to the ground at an angle and then connect it to a speedometer. Accuracy is usually within 1-3% up to 300mph.

[ollie87]

I believe the sensor in F1 is actually measuring the ride height of the vehicle.

[bonzini]

In some jurisdictions the law says that the car speedometer must show a higher speed than the correct one (as long as tyres aren't above the correct pressure), so that the driver can "trust" the speedometer to avoid them a speeding ticket. On my car 140 km/h on the speedometer corresponds to 128-130 km/h on the GPS.

[Scoundreller]

They probably use GPS? If not, then just get bigger tires.