# how does a car know how fast it’s going?

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how does a car know how fast it’s going?

In: Technology

There is a monitor that counts the rate of rotations at the output end of the transmission. There is a known ratio between rotations there and rotations of the drive wheels, and there is an expected diameter for each drive tire. Based on these parameters, a computer calculates what the speed must be.

If i continued telling you more, it would be a VERY long paragraph

There are a few ways. The most common currently is that each wheel has a sensor that measures how fast it it rotating. One of the control modules receives these signals and averages the rotation speed of the driven wheels. The module will have been configured with its tyres’ rolling circumference when it was built. The module multiplies this circumference by the rotation speed to calculate the vehicle speed. Some signal processing will be done to filter or any noise on the calculation.

Before the computer era: A gear on the transmission drives a wheel. The wheel spins a magnet, the spinning magnet induces an electrical current, the current causes the speedometer needle to move. This can be tuned based on the size of the tires by changing the tension on the spring that holds the speedometer at zero or electronically by adjusting the resistance on the circuit.

Today, the voltage measured by computers, but the basics are the same.

tl;dr: Magnets!