Like, how do they work? They’ve been around since nearly as long as cars have been around so it’s not some fancy modern engineering. How does it know what speed the cars going?
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It’s surprisingly simple in concept.
Your tires are a certain size. They can’t spin unless you’re moving. One full rotation is a set distance traveled. It’s easy to tell how fast they’re spinning. From there it’s just a ratio to tell how fast you’re moving. You can do it with a purely mechanical linkage and gearbox, no electricity required, much less computers.
If you’re thinking “Wait, but as the tires wear down the diameter changes, so how does it remain accurate?” then you’re 100% right! Your speedometer is not accurate. But it’s close enough.
There may be newer cars that will use more high tech methods (like radar or GPS) to get a more accurate measure.
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