If you increase the elasticity of a material, what happens to the speed of a mechanical wave going through it?

At the molecular level, materials are held together by bonds, which act like springs for small displacements from the equilibrium spacing between neighboring atoms. Push the atoms close, the bond pushes back to keep them apart. Pull them apart, the bond pulls the atoms closer. For those small displacements, it acts like a spring

The speed of the wave will be related to the stiffness of of those springs - you compress the material - how quickly do all of those little springs rebound and push their neighboring atoms away, sending that wave of compression through the material.

Explanation:

prolly a beacuse the momentuem the car has increases while going down hill