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In basic thermodynamic calculations, gasoline is commonly approximated as octane (C₈H₁₈), for which the stoichiometric air-fuel ratio (AFR) is 15.1. However, in automotive applications involving the control of spark-ignition engines, the AFR for gasoline is typically assumed to be 14.7. This value reflects the average hydrocarbon composition of commercial gasoline, which is a complex mixture of various hydrocarbons including alkanes, cycloalkanes, and aromatics. The averaged elemental composition of gasoline is often expressed as C₁H₁.₉₅, which does not correspond to a specific molecule but adequately describes the bulk properties of the fuel. A similar issue arises when determining the AFR for commercially available fuels such as E5 and E10, which are mixtures of gasoline and ethanol with 5% and 10% ethanol content by volume, respectively. Using a simplified volumetric approach, the AFR for E5 is approximately 14.4 and for E10 about 14.1. However, because ethanol is denser than gasoline, volume-based calculations do not accurately reflect the true stoichiometry. The mass-based ethanol content increases by approximately 6.5% for E5 and 13% for E10. Taking into account the averaged hydrocarbon composition of gasoline and the mass-based ethanol content, the AFR values can be adjusted to 14.33 for E5 and 13.96 for E10. The aim of this article is to determine correction coefficients for exhaust gas analyzers or fuel-air mixture control systems in vehicles. The study compares two methods for evaluating the air-fuel ratio (AFR): one based on the oxygen content in exhaust gases using a wideband exhaust oxygen sensor, and the other based on the concentrations of exhaust gas components using a simplified Brettschneider equation. The latter method analyzes the following components in the exhaust gases: oxygen (O₂), carbon monoxide (CO), carbon dioxide (CO₂), and hydrocarbons (HC).