The rate of diffusion of a gas is inversely proportional to the square root of its molar mass. The molar mass of methane (CH₄) is 16 g/mol, while the molar mass of carbon dioxide (CO₂) is 44 g/mol.

Therefore, methane will diffuse faster than carbon dioxide since it has a lower molar mass.

$$rate \propto \frac{1}{\sqrt{M}}$$