  Weyl fermions with tilted linear dispersions characterized by several
different velocities appear in some systems including the quasi-two-dimensional
organic semiconductor $\alpha$-(BEDT-TTF)$_2$I$_3$ and three-dimensional
WTe$_2$. The Coulomb interaction between electrons modifies the velocities in
an essential way in the low-energy limit, where the logarithmic corrections
dominate. Taking into account the coupling to both the transverse and
longitudinal electromagnetic fields, we derive the renormalization group
equations for the velocities of the tilted Weyl fermions in two dimensions, and
found that they increase as the energy decreases and eventually hit the speed
of light $c$ to result in the Cherenkov radiation. Especially, the system
restores the isotropic Weyl cone even when the bare Weyl cone is strongly
tilted and the velocity of electrons becomes negative in certain directions.
