Question

The instantaneous speed is always equal to the magnitude of instantaneous velocity for an object moving along a straight line, why?

Answer 1

When a body moves along a straight line then the distance and the displacement of the body is equal.
We know that 
           Velocity = displacement/time
            speed = distance/time
when the value of distance and displacement of the body is equal  then 
             velocity=speed
Hence the magnitude of velocity is equal to the magnitude of the speed.

Answer 2

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Instantaneous velocity. The rate of change of displacement of an object in a particular direction is its velocity.The magnitude of instantaneous velocity equals the instantaneous speed. This happens because, for an infinitesimally small time interval, the motion of a particle can be approximated to be uniform.