A car speedometer tells the driver how fast the vehicle is moving by turning rotation into a speed reading. In most modern cars, this starts with a sensor that detects how fast a wheel or transmission output shaft is spinning. The system is important because speed affects safety, braking distance, fuel use, and legal driving limits.
A clear speed display helps the driver make quick decisions without needing to calculate motion directly.
Key Facts
- Vehicle speed comes from rotation rate and tire circumference: v = fC
- Tire circumference is C = 2πr, where r is tire radius.
- If wheel rotation is measured in revolutions per second, speed in m/s is v = revolutions per second × circumference.
- Many digital sensors create pulses, so rotation rate can be found with f = pulses per second ÷ pulses per revolution.
- To convert m/s to km/h, multiply by 3.6: speed in km/h = speed in m/s × 3.6.
- To convert m/s to mph, multiply by about 2.237: speed in mph = speed in m/s × 2.237.
Vocabulary
- Speedometer
- A speedometer is the dashboard instrument that displays a vehicle's current speed.
- Vehicle Speed Sensor
- A vehicle speed sensor is an electronic device that detects rotation of a wheel, axle, or transmission shaft and sends a signal to the car's computer.
- ECU
- The ECU, or electronic control unit, is a car computer that reads sensor signals and calculates values such as vehicle speed.
- Pulse Signal
- A pulse signal is a repeating electrical on and off pattern that a sensor can produce as a rotating part passes by.
- Tire Circumference
- Tire circumference is the distance a tire travels in one complete rotation.
Common Mistakes to Avoid
- Using tire diameter instead of circumference in v = fC is wrong because one wheel rotation moves the car forward by the tire circumference, not the diameter.
- Forgetting to convert minutes to seconds gives the wrong speed because rpm must be converted to revolutions per second before using v = fC in meters per second.
- Assuming the speedometer measures engine speed is wrong because engine rpm changes with gear ratio, while road speed is usually based on wheel or transmission output rotation.
- Ignoring tire size changes can make predictions inaccurate because a larger or smaller tire travels a different distance per rotation.
Practice Questions
- 1 A tire has a circumference of 2.0 m and rotates 10 times each second. What is the car's speed in m/s and km/h?
- 2 A speed sensor produces 48 pulses per wheel revolution. If the ECU counts 720 pulses each second and the tire circumference is 2.1 m, what is the vehicle speed in m/s?
- 3 A driver replaces the original tires with larger tires but the speedometer is not recalibrated. Explain whether the speedometer is likely to read too high or too low, and why.