What Is a Speedometer? How It Works and Why It Matters

A speedometer is an instrument that shows how fast a vehicle — or a person — is moving at this moment. In a car it’s the dial or digital number right behind the steering wheel. On a phone it can be an app or even a web page. Whatever form it takes, it answers one question: how fast am I going right now?

What a speedometer measures

Speed is distance traveled divided by the time it took. A speedometer shows your instantaneous speed— your speed over the last fraction of a second — rather than your average speed for a whole journey. It’s usually shown in miles per hour (mph) in the US and UK, kilometers per hour (km/h) in most other countries, and knots on boats and aircraft.

Speedometers are often confused with two neighbors on the dashboard:

• Odometer — counts the total distance a vehicle has traveled (the mileage).
• Tachometer — shows how fast the engine is spinning, in revolutions per minute (RPM), not how fast the car is moving.

The three main types of speedometer

1. Mechanical (eddy-current) speedometers

For most of the 20th century, cars used a mechanical speedometer. A flexible cable, turned by the transmission, spins a magnet inside the instrument. The spinning magnet creates eddy currents in a metal cup attached to the needle, dragging it around against a small spring. The faster the magnet spins, the further the needle moves. German engineer Otto Schulze patented this design in 1902, and it stayed standard for decades.

2. Electronic speedometers

Modern cars measure speed electronically. Sensors on the transmission or at the wheels (the same ones the anti-lock brakes use) produce pulses as they turn. The car’s computer counts the pulses, multiplies by the distance each wheel turn covers, and drives a digital display or a motorized needle.

Both mechanical and electronic speedometers share one limitation: they measure how fast the wheels turn and assumea certain tire size. Change tire size, let tires wear down, or run them under-inflated, and the reading drifts. That’s one reason car speedometers usually read a little high.

3. GPS speedometers

A GPS speedometer ignores the wheels entirely. It measures speed using signals from navigation satellites, so it works the same in a car, on a bike, on foot, on a boat, or in an airplane seat. Every modern smartphone has a GPS receiver, which is why a phone can act as a speedometer. We explain the details in how a GPS speedometer works.

How accurate are speedometers?

Why speedometers matter

• Safety. Speed is a factor in a large share of serious crashes. Knowing your speed is the first step to controlling it.
• Staying legal. Speed limits are enforced to the mph. A reliable reading helps you avoid tickets.
• Fuel economy. Most cars use noticeably more fuel at high highway speeds. Holding a steady, moderate speed saves money.
• Training and performance. Cyclists, runners, and drivers use speed data to track progress — for example, timing 0–60 mph.

Do you need a separate speedometer?

If you drive a car with a working speedometer, you already have one. But a second, independent reading is useful more often than you’d think: checking how far off your car’s dial is, riding a bike or e-scooter, boating, or driving an older or rental vehicle. A phone-based speedometer also adds things a dashboard can’t, like trip history, speed alerts, and a windshield HUD. See 8 reasons to use a speedometer app.

Summary

• A speedometer shows your current speed, in mph, km/h, or knots.
• Car speedometers measure wheel rotation; GPS speedometers measure movement using satellites.
• Car speedometers usually read slightly high; GPS is typically more accurate in open sky.
• You can try a free GPS speedometer in your browser right now — open the online speedometer.