How does a car demonstrate the physics concept of speed

Cars can
Accelerate, Decelerate, and not move(when there parked)

The formula for speed is equal to distance/time (v=d/t). That is why speed is measured in k/hm (v=km/h). A car can travel x amounts of distance over y amounts of time, so it demonstrates speed perfectly. You can just drive somewhere, see how many kilometres you drove for and count the time, and you'll be able to find speed. Also, cars have an ability to accelerate and decelerate. The formula for acceleration is a=v2-v1/t. In this, speed is involved, meaning the cars acceleration is related to speed as well. Cars demonstrate the concept of speed in a simple way.

It moves. And in that case the distance it travels is non-zero. And it doesn't arrive at its destination instantly so it must take some time. Dividing the distance a car travels by the time it takes to arrive at its destination gives you the average speed of the car.