The rest energy is
The kinetic energy is
The total energy is therefore
The momentum is found by
The effective mass is
The speed is therefore
The classical calculation is much simpler:
Classical mechanics gives a speed which is more than twice as large as relativity. This result is not just quantitatively wrong: Relativity theory tells us that no particle can be accelerated beyond the speed of light. (Try repeating the calculation with larger and larger kinetic energies if you need to convince yourself of this.) Classical mechanics does not and cannot predict this effect.