Eli5, How do speedometer know how fast the car is going?


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?

In: 2

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.

Modern speedometers are somewhat of a bit of fancy modern engineering. You have a sensor which give a pulse every time a teeth in a cog passes and then there is a microcontroller counting these pulses and calculates the speed which it then use to send signals to the small servo motor in your speedometer dial.

Old speedometers though worked a bit different. There were a cable connected to the output shaft of the transmission all the way to the dial. This cable would then spin with the wheels. In the dial this cable is connected to a magnet which is therefore also spinning. Over this magnet there is a cup, which is eventually connected to the needle. The magnet and the cup does not touch directly but are magnetically coupled so when the magnet moves the cup also moves. However there is a spring pushing the needle back to zero. The faster the magnet moves the more it pushes on the cup and the further the spring is tightened pushing back against the magnet. You can read off the speed from the dial.

There are several ways but the most basic is “look at the wheel axle rotation”. If you know the diameter of the wheels, and the wheels aren’t slipping, you can easily calculate the forward speed from the rotation speed. It’s a simple formula: for every wheel turn the car moves [circumference of a wheel] forward.

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.


For each 1/8 of an inch of tire wear, the speedometer will show you going about 1 MPH faster than you’re actually going. IF I did my math correctly.

Not a big deal unless you go from smaller-to-larger tires. E.g., 19″ to 20″ which could have you trying to explain math to a State Trooper about how you double the wear because it’s worn on both the top AND bottom so the circumference and so on and what not until he drags you out and starts clubbing you with his nightstick.