The average speed is therefore a scalar . In a time interval of 5 minutes, a runner runs once around a one-mile track. After 5 minute the runner returns to his starting position. The displacement is zero, so his average velocity is zero. This distance is one mile.
Speed is a scalar, velocity is a vector.
The average speed is in general not equal to the average velocity. A sprinter runs north in a straight line and covers a distance of 100m in 12s. P 2 - P 1 is 100m north. For objects moving along a straight line without reversing their direction, the average speed is the magnitude v of the average velocity v . Often the average velocity is not a very useful quantity.