Velocity difference of the MC front between normal and inverse CMEs with the same initial velocity ejected in the same background wind vs. time. The colours represent different initial velocities (green: 400 km s⁻¹, red: 800 km s⁻¹, blue: 1200 km s⁻¹), while the line styles represent different background wind densities (dashed: 4 cm⁻³, solid: 8 cm⁻³). The black dotted line represents v^diff = 0.