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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Say your front wheels are 20 inches in diameter. One rotation of the wheels covers 20π inches, which is about 63 inches. So if your wheel spins ten times every second, the speedo knows you are going 630 inches per second. From there it’s just unit conversion to get to 36 miles per hour.
One side effect of this is that if you put bigger or smaller wheels on your car, your speedo will be wrong unless you recalibrate it.
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