Similarity: Speed and velocity both represent a way to measure the change in position of an object relative to time. in fact for a straight line motion the speed and velocity of an object are the same .speed and velocity are measured in the same unit Difference: Speed is the ratio of the distance travelled over the time taken to travel .it gives us an idea of how fast an object is moving .it is a scaler quantity .that is it only have a magnitude but no direction Velocity r is the ratio of displacement and the time taken for this motion .it gives us an idea of how fast the object is changing its position. velocity is a vector quantity and has both magnitude and directio