what is drift speed???

The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. In general, an electron will 'rattle around' randomly in a conductor at the Fermi velocity. An applied electric field will give this random motion a small net velocity in one direction.

The formula for evaluating the drift velocity of charge carriers in a material of constant cross-sectional area is given by:

v = I \ nAq

where v is the drift velocity of electrons, I is the current flowing through the material, n is the charge-carrier density, A is the area of cross-section of the material and q is the charge on the charge-carrier.