In a modern car, your speedometer might look analog, but it is almost certainly digital and driven by the computer that has to monitor all sorts of things anyway. But how did they work before your car was a rolling computer complex? The electronic speedometer has been around for well over a century and, when you think about it, qualifies as a technlogical marvel.

If you already know how they work, this isn’t a fair question. But if you don’t, think about this. Your dashboard has a cable running into it. The inner part of the cable spins at some rate, which is related to either the car’s transmission or a wheel sensor. How do you make a needle deflect based on the speed?

Mechanical Solutions

Early versions of the speedometer used a governor pulling against a spring. The faster it rotates, the more the two weights of the governor pull out against the spring, and the needle moves with the weights.

As an aside, this sort of centrifugal governor is also known as a fly-ball governor, and similar devices were commonly used to regulate the maximum throttle on steam engines. The arms of the governor would be fully extended once the engine reached its top speed, which lead to the term “balls-out” becoming used to describe a machine operating at its upper limits.

Another type of mechanical speedometer had an escapement like a watch. The time mechanism would move the needle back, and the rotation of the wheels would move it forward. The net result was a needle position that would increase with speed.

The Magnetic Approach

However, most cars use a magnetic type speedometer — although it doesn’t work in the way you might imagine. There’s no reed relay or Hall effect sensing the magnetic field. Instead, there is an aluminum cup attached to the speedometer needle and, nearby, a magnet that spins on a shaft moving at some ratio of the car’s speed. There’s no direct connection between the two.

Being a non-ferrous metal, aluminum is not generally something we think of being affected by magnets. Under normal circumstances that might be true, but a moving magnetic field will induce eddy currents in aluminum. This forms a field in the aluminum, too, and the spinning magnet tends to drag the cup, thereby deflecting the pointer.

A spring similar to one you might find in a mechanical clock or watch pulls back the pointer so the needle hovers at the point where the force of the magnet pulls against the spring. The pull on the spring has to account for the gear ratios and the size of the tires to accurately reflect the vehicle’s speed.

If you want to see an entertaining teardown of an old speedometer, [Tubalcain/Mr Pete] has you covered in the video below. He also shows how the odometer part worked, too.

Modern Times

Of course, these days you are more likely to pick up a pulse using a Hall effect or some other part of the vehicle and just count the pulses in the car’s computer. In fact, the pulses might be encoded at the source and travel over something like a CAN bus to get to the computer.

It is also possible to pick up speed from other tracking information like GPS, although that might not be as accurate. But if you have, for example, a mobile phone app that shows your speed, that’s probably what it is doing. The obvious way to do that is to take position measurements periodically and then do the math. However, more sophisticated systems can actually measure Doppler shift to get a more accurate reading.

We see a lot of bicycle speedometers for some reason. Eddy currents make induction cooktops work, too. Even tiny ones.