Speed is defined as a measure of the distance an object travels in a given length of time.
The average speed of an object can be determined from the equation below:
Thus, if a car travels 100 meters in 5 seconds its average speed can be calculated as;
Average speed = 100/5 = 20 meters per second
Average speed is used to give the speed of an object over a given interval of time, if however the speed of an object is required for a particular moment then the instantaneous speed is used.
Instantaneous speed is the speed of an object at a given moment. In this case the equation is similar to that of average speed but the time taken is a much smaller interval. A speedometer in the car gives an instantaneous speed as it gives the speed of the moving car at that specific time, whereas the average speed would be used as a measure of speed for the whole journey.