1. Speedometer: This is the most common way to measure speed. It's a gauge in the car that displays the vehicle's instantaneous speed in units like miles per hour (mph) or kilometers per hour (kph).

2. GPS: GPS devices and apps use satellite signals to determine your location and calculate your speed based on the distance traveled over time. This is often more accurate than a speedometer, especially at higher speeds.

3. Radar Guns: Law enforcement uses radar guns to measure the speed of vehicles. These devices emit radio waves that bounce off the moving vehicle, and the frequency shift of the reflected waves reveals the vehicle's speed.

4. Laser Speed Guns: Similar to radar guns, but use laser beams instead of radio waves. They are very accurate and are often used by traffic enforcement.

5. Timing Devices: For measuring speed over a known distance, you can use a stopwatch and calculate the speed by dividing the distance by the time taken. This method is often used for racing and track events.

It's important to note that:

* Speedometers can be inaccurate: They are often calibrated for a specific tire size and can become less accurate with wear and tear.

* GPS speed can fluctuate: Signal strength and obstructions can affect the accuracy of GPS speed readings.

* Radar and laser guns are considered the most accurate: But they can be affected by weather conditions.

So, the way the speed of a car is measured depends on the purpose and the available tools.