Electron drift velocities obtained by simulation of Schlumbohm's and Frommhold's experiments using a Monte Carlo method are compared with theoretical electron drift velocities such as the diffusion modified drift velocity V_d, the average drift velocity of electrons W_v, the center of mass drift velocity W_r and the mean arrival time drift velocity W_m, when electron impact ionisation is present. The calculation is carried out using a constant collision frequency model gas for reducing the computational time. It is found that the electron drift velocities calculated by simulating Schlumbohm's and Frommhold's experiments do not agree with any of the theoretical electron drift velocities described above, but coincide with a drift velocity represented as (W_r+W_m)/2.