Speed is the rate of change of motion, i.e. distance moved by an object in a specified time irrespective of direction. Velocity is speed with respect to direction. Speed is a scalar quantity while velocity is a vector.

Comparison chart

Speed Velocity Scalar Vector Distance Displacement Distance, time Distance, time and direction of motion Distance/time Displacement/time
A shot from the inside of a speedy car

Calculation

Speed is the rate at which an object covers a specified distance. It is a scalar quantity in the sense that direction is not required to define speed. If an object doesn’t cover any distance at all then its speed is zero.

Instantaneous speed is the speed at any given instant of time. Average speed is the average of all instantaneous speeds or distance per time ratio.

$Average\ Speed = Distance\ travelled /Time\ taken$

Velocity is the rate at which an object changes its position. It is a vector quantity. The direction of velocity is the direction in which the object is moving. The absolute value or the magnitude of velocity is speed. Velocity can also be defined as the displacement of an object in a specified direction. If the object returns to its starting point then its velocity is zero. Instantaneous velocity is the velocity of an object at any given instant in time. Average velocity is the displacement per time ratio.

$Average\ Velocity = Displacement/Time\ taken$

The rate of change of velocity with respect to time is called acceleration. To maintain a constant velocity, the object has to travel at a constant speed in a constant direction. When an object is constrained to move in a constant direction it moves along a straight line. When there is a change in speed or direction, the object is said to be accelerating.

Example

The following video illustrates how to calculate average velocity and speed:

Measurement

Both speed and velocity are kinematic quantities and measured in miles/hour, kilometers/hour or meter/second.

Applications in real life

Average speed is usually used to estimate the amount of time taken to cover a distance between specific points while driving. A speedometer shows the instantaneous speed of a vehicle.

Velocity is used to calculate the time taken for a storm to reach the coastline, time taken for rockets to reach the moon and so on.