how does a car speedometer work

[PatrickLynn]

A car speedometer works by measuring the rotational speed of your vehicle's wheels and converting that data into a speed reading displayed on your dashboard. Modern cars primarily use an electronic sensor, often called a vehicle speed sensor (VSS), mounted on the transmission. This sensor reads a gear or a tone ring that rotates at a speed proportional to the wheels. It generates an electronic signal, which is sent to the car's computer. The computer then calculates the vehicle's speed based on the signal frequency and the programmed tire size, finally sending the information to the gauge or digital display in your dashboard.

While this is the standard method today, the technology has evolved. Older vehicles used a purely mechanical system involving a flexible cable spun by the transmission. The core principle remains translating wheel rotation into a speed value, but electronic systems are far more accurate and reliable.

The accuracy of this reading is crucial and depends on the tire size programmed into the computer. If you install larger or smaller tires than the factory specification, your speedometer will show an inaccurate speed. This is because the computer is still calculating speed based on the original tire's revolutions per mile. For example, larger tires cover more ground per revolution, causing the speedometer to read slower than your actual speed.

Here’s a quick comparison of the two main systems:

Feature	Mechanical Speedometer	Electronic Speedometer
Data Source	Flexible spinning cable driven by transmission	Electronic signal from a Vehicle Speed Sensor (VSS)
Signal Path	Direct physical connection	Wires to the car's computer (ECU)
Common Issues	Cable wear, breaking, gauge mechanism failure	Faulty sensor, wiring problems, computer errors
Accuracy	Less accurate, prone to fluctuation	Highly accurate and stable
Tire Size Impact	Directly affected by changes	Affected if ECU programming isn't updated
Typical Vehicle Age	Pre-1990s vehicles	Most vehicles from the 1990s to present

[Ezekiel]

Think of it like a watch for your wheels. There's a little sensor on the transmission that counts how fast your wheels are spinning. It sends that info to the car's computer, which does the math to figure out how fast you're going. It's pretty smart, but it's calibrated for your original tires. If you put bigger tires on your truck, for instance, the speedometer will think you're going slower than you really are because each wheel rotation covers more distance.

[MadelineDella]

It's all about counting rotations. My old '73 Chevy had a cable that literally spun around from the transmission to the back of the gauge. You could sometimes hear it whining. Newer cars are simpler and more reliable. They use a magnetic sensor that picks up a signal from a gear in the transmission. No cables to break. The computer just reads the pulses and moves the needle on the dash. The basic idea is the same, but the execution is much cleaner now.

[StPeter]

From an engineering perspective, the system is elegantly simple. A reluctor ring (a toothed wheel) rotates with the transmission output shaft. A stationary sensor detects each passing tooth, creating a digital square wave signal. The frequency of this wave is directly proportional to rotational speed. The Engine Control Unit (ECU) converts this frequency into a speed value, referencing a pre-programmed tire circumference. Any deviation from this circumference, like tire wear or aftermarket sizes, introduces a calculable error into the reading.

[DeLeo]

I was curious about this after my son asked. It's not magic; it's a clever sensor. A device near your wheels sends electronic pulses to the car's main computer. The computer knows how far the car should travel with each wheel turn based on the original tire size. It just counts the pulses per second and calculates the speed. That's why it's important to get your car's computer updated if you change to a significantly different tire size, to keep the reading accurate and safe.