The present work aims to measure speeds of sound c in a biogas mixture of CH4+N2+CO2+CO, at p = (1 to 12) MPa and T = (273, 300 and 325) K, using a spherical acoustical resonator. The results are fitted to the virial acoustic equation of state, and the virial acoustic coefficients are obtained, β_a and γ_a and extrapolated to zero pressure, determining the adiabatic coefficient as perfect gas, γ^pg, and the isobaric and isochoric heat capacities as perfect gas, C_p^pg and C_V^pg, respectively. The speeds of sound are acquired with a mean expanded relative uncertainty of 165 parts in 106 (k = 2) and are compared with the results predicted by the reference equation of state for this kind of mixture (natural gas-like mixtures), EoS GERG-2008. Relative deviations between experimental data and values estimated by this model were less than 700 parts in 106 at T = 325 K, and below 400 parts in 106, and within measurement uncertainty of at T = 300 K, although appreciably higher at isotherm T = 273 K at the highest pressure data for this work, and even reaching values above 3.400 parts in 106.
