A comprehensive analysis of the autoignition tendency of binary mixtures of hydrogen and carbon monoxide diluted with carbon dioxide is carried out in this paper. Methane Number (MN) measurements of these mixtures were performed by using a method analogous to the standard method for Octane Number (ON) measurement. The experimental MNs ranged from 46.0 to 139.1 according to experimental data. These results are compared with the MNs calculated by programs released for natural gas MN estimations. A comparative analysis concluded that these programs are not suitable to calculate MN of syngas. A method to isolate the effect of carbon dioxide on the autoignition tendency of binary mixtures of hydrogen and carbon monoxide is proposed. Results reveal that increments of carbon dioxide concentration in the mixtures, exponentially increase the MN of the mixtures. At low carbon dioxide concentration, the MN of the ternary mixtures can be approximated by the MN of their equivalent binary mixtures of hydrogen and carbon monoxide. Based on this observation, the MNs of coal gases reported in the literature are estimated.