[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.