  Three-dimensional track reconstruction is a key issue for directional Dark
Matter detection. It requires a precise knowledge of the electron drift
velocity. Magboltz simulations are known to give a good evaluation of this
parameter. However, large TPC operated underground on long time scale may be
characterized by an effective electron drift velocity that may differ from the
value evaluated by simulation. In situ measurement of this key parameter is
hence a way to avoid bias in the 3D track reconstruction. We present a
dedicated method for the measurement of the electron drift velocity with the
MIMAC detector. It is tested on two gas mixtures : $\rm CF_4$ and $\rm
CF_4+CHF_3$. We also show that adding $\rm CHF_3$ allows us to lower the
electron drift velocity while keeping almost the same Fluorine content of the
gas mixture.
