A car’s speedometer measures the instantaneous speed of the vehicle, typically displayed in miles per hour (mph) or kilometers per hour (km/h).

Quick Scoop

  • A speedometer shows how fast you are going right now , not your average over a trip.
  • In most modern vehicles, it displays speed in mph, km/h, or both at the same time.
  • It does this by sensing how fast components like the driveshaft or wheels are rotating and converting that into road speed.

Instantaneous vs average speed

  • Instantaneous speed is the speed at a specific moment in time, which is exactly what the speedometer indicates as the needle or digits change continuously with your motion.
  • Average speed would be total distance divided by total time for a trip, which is not what the speedometer shows; that is instead inferred from an odometer or trip computer over time.

How the number is produced

  • Mechanical speedometers use a cable from the transmission that spins in proportion to vehicle speed; this motion is translated into a needle position showing speed.
  • Electronic speedometers use sensors to detect rotational speed and an internal computer to convert that data into a speed reading in mph or km/h.

Information gathered from public forums or data available on the internet and portrayed here.