Measurement of cetane number (CN) of a mixture of fatty acid methyl esters (FAME) [biodiesel] from a large number of lipid sources is expensive. This has resulted in cetane index (CI) estimated by a proposed equation derived from three simultaneous equations, reported in literature as cetane number (CN). Since this proposed equation results in a derived number, this study compares CI reported as CN by this equation with derived cetane number of a mixture of FAME (biodiesel) present in five non-traditional oil seed samples. Derived cetane number (DCN) of a mixture of FAME for each non-traditional oil seed sample is calculated from the measured derived cetane number of each FAME of 99% purity totalling 100%. Percentage relative error (% RE) between reporting of CI as CN by the proposed equation and DCN of a mixture of FAME of five non-traditional oil seed samples is >15 and increases with increasing percentage of saturated FAME. This study concludes that it is erroneous to report CI as CN by the proposed equation.